Yousaf, currently Minister for External Affairs and International Development, says an SNP government after the referendum would make Scotland a "world leader in international development".
He said: "The aim should be, when independence comes, hopefully if it does come in 2014 - we would essentially go from having a budget as we have now of £9 million, modest, to a budget of hundreds of millions."
If there is a Yes vote in 2014, he says, a future SNP government would aim not only to hit the UN target of dedicating 0.7% of gross national income to aid, but go beyond that, possibly to 1%.
The 0.7% target has not been reached by the UK Government for the past 40 years.
The SNP is also "actively considering" cancellation of third world debt. Assuming a Yes vote for independence, he says Scotland would have an aid budget of hundreds of millions per year.
Yousaf, the first Scottish minister to take on a foreign portfolio, says he wants "a unique Scottish voice on the international stage", adding: "We want people to know that Scotland has a role to play in external affairs and foreign policy. It has a unique perspective. We are showing our political intent."
Scotland is already showing it is following a different international agenda to the UK Government. Last year, during the UN vote on whether to give Palestine upgraded diplomatic status, the UK Government abstained, but Yousaf wrote to the Foreign and Commonwealth Office, asking them to reconsider.
Being a small country, says Yousaf, should be no barrier to Scotland becoming a leader in global affairs: "Many world leaders are small countries. Norway has a fantastic reputation for international development. If you speak to some of the academics in this field, they'll tell you it's much easier to be a smaller country and make it work than a very big country. A country the size of the US, or even the UK, has a lot of commitments because of its geopolitical position in the world."
Scotland currently targets aid on just eight countries, which are, Yousaf points out, "some of the poorest in the world". Among them are Malawi, Zambia, Rwanda and Tanzania, some which have a long history with Scotland. The nation's links with Malawi, for instance, date back to David Livingstone, whose bicentenary is celebrated this year. The plan is that the profile of countries given aid will remain similar in the future.
However, it will be continually reviewed according to the UN human development index.
In the past year, the Scottish Government has ceased funding projects in Sri Lanka, and limited its India funding to only the three poorest states. There has been criticism in the UK of aid to India due to the burgeoning wealth of the country.
On the issue of expanding the Scottish aid budget, Yousaf said: "We've already passed a resolution at our last parliamentary conference to say that, not only will we ensure that we meet the 0.7% [UN spending target], but that we'll look to go to 1%. Because we don't want to stop there, we want to be more aspirational. There are some European countries that do that, they hit 0.9%, 0.95%, touching 1%."
But rather than increase the number of countries helped, Yousaf's intention is that aid remains targeted and "focused". "The temptation would be to think that given we're now working in seven or eight countries, let's work in 70-80 countries. But that's not the way to do things."
"Climate justice" – political-speak for wealthy nations taking responsibility for some of the effects of climate change in poorer countries – is something in which Scotland has already taken a global lead, Yousaf says.
The Scottish Government's "climate justice fund" is the first of its kind in the world. In December, the Minister for Environment and Climate Change, Paul Wheelhouse, announced that Scotland would host an international conference on the subject. Much of the international development funding is already dedicated to projects relating to sustainability, food security and climate impact.
"Building hospitals and schools is important," said Yousaf, " but as a government what you want to do is try to use your country and nation's expertise to make a sustainable long-term development difference. And that's what Scotland will concentrate on."
With this year's G8 summit taking place in the UK in June, poverty campaigners have called on the Scottish Government to cancel its share of third-world debt. This is something, said Yousaf, which the SNP is "actively considering" post-independence.
He added: "In the year of the G8 there is a huge opportunity to be able to push a groundswell on this, to think radically and take the public with you."
Opposition parties said last night that, while the SNP's aid ambition is laudable, it needed to be fully costed, with details of which other budgets might need to be cut to pay for it.
Patricia Ferguson, Labour's external affairs spokeswoman, said: "The SNP find it easy to make grand boasts about what they would spend in a separate Scotland but are woefully short of detail and to make empty promises to some of the poorest people in the world is simply irresponsible."
Scottish Liberal Democrat leader Willie Rennie said: "To fund their decision to maintain a foreign aid budget of 0.7%, we'd be faced with cuts elsewhere or higher taxes - The Nationalists need to get the sums right before making giant promises."
A Scottish Conservative spokesman said: "It is all very noble for the SNP to make a £1 billion pledge on foreign aid but again there is no detail on how they would fund this."
Humza Yousaf became an activist at the age of 12 in the basement of the Scottish Islamic Relief Office. It was during the summer holidays and his mother insisted he work in his accountant father's office, but he could think of "nothing worse" than working through VAT returns all summer. His mother suggested his only option would be to work, just across the road, in Islamic Relief, and he went along and was put to work, clearing out a basement of campaign materials. What he saw about global poverty inspired him.
"The Iraq war was the trigger for my politics, but if we're talking about activism, that was the start." And the same spark still drives Yousaf now, as Scotland's only Muslim MSP and first Minister of External Affairs and International Development. He says: "There are not many people my age, 27, who can say they're living their absolute dream job."
However, it's a dream job which has led him to wake up almost every night with a knot in his stomach. "Sometimes you can't sleep at night, because of your concern about something you saw or read." It can be a trial for his wife, SNP activist Gail Lythgoe.
"Anyone who has been involved in international development," he adds, "be it rattling a tin or campaigning, will tell you it's very difficult not to get more and more involved."
For many years Yousaf was the volunteer media officer for Islamic Relief and saw the human impact of many disasters.
Because of his own background, he also feels a connection to the wider world. The son of a Pakistani father and a mother who grew up in Kenya, but was one of the Asian migrants forced to leave, he adds: "I'm fiercely proud of being Scottish but just as proud of being Pakistani."
Anne Mutata, Zambia
Mutata recalls a time when she didn't have enough food to feed her family of four children and six orphaned nieces and nephews. "Sometimes we had enough to eat, sometimes we had nothing," she says. "Before I went to college, I couldn't make compost and the crop yield was very poor." Now she has been elected by her community to serve as lead farmer in the organic farming programme, heading their study circle and supporting villagers to learn new skills.
She has been helped by the Kulima Sustainable Rural Food Security Programme, a collaborative initiative, funded by the Scottish Government and implemented by Scottish Catholic International Aid Fund [SCIAF]. It promotes sustainable agriculture techniques which can increase soil fertility and food production, and counteract climate change. The word "Kulima" is very common in the Bantu languages spoken in eastern and southern Africa, and means 'to till'.
Mutata has also established a co-operative, mostly of women. Since its founding, 18 farmers have joined and she has high hopes the figure will soon reach 30. She is the only female farmer to sit on the board of the Kasisi Training Centre, a farmer training institute near Lusaka. She represents her peers and female household heads. Organic farming has changed her life. "Now," she says, "with money from cash crops, I can pay for my children to go to school."
Exaveria Sangijo, Northern Tanzania
Sangijo is a farmer from Chela in Shinyanga. She is married with seven children and is taking part in a Scottish Government-funded Oxfam project to help farmers adapt to climate change.
"One of the biggest challenges we face is growing rice when there is a shortage of water," she says. "The rains have not been enough. We are not able to grow rice as we should be. When there is plenty of water we produce enough but, of course, when there is a shortage we don't produce that much. We rice farmers depend on the rain."
Naturally, she wonders why it is the rains sometimes don't come. "If it is the factories [in developed countries] that are contributing to the shortage of rains, they should do whatever it takes to ensure that the rains keep on coming."
The project is designed to produce greater food security. "Our lives are improving through this project," she says. "We are learning how to increase the yield from the fields through better quality seeds, irrigation and planting methods. This Oxfam project is helping us by moving us from where we were to a better place. Now a lot of families have a better life – we are able to build houses and educate our children. I used to live in a grass-thatched house. Now I have constructed a brick house with iron sheets. Now I can take my children to school – something that I never thought I would."
Elizabeth Phiri, Malawi
Phiri, 36, is a farmer in the village of Msambo in Lilongwe. She is a member of her Village Farmers' Forum, set up with support from the Scottish Government's Malawi Fund. Before joining, the mother of four lived in poverty. In a male-dominated society, money from her husband was not forthcoming, despite her contribution to the family farming business. "I used to spend most of my time doing household chores," she recalls. "In addition, I was assisting my husband in the field. I was also responsible for the production of household food."
Phiri's story is like that of many women in rural Malawi. The men take the produce to market and keep the proceeds. The women stay at home and get what the men give them. But with the establishment of the forum in her village, Elizabeth learned about her rights. "My husband and I joined the forum," she says. "During the meetings, women shared our fears, aspirations, interests and challenges: having to labour for the family while our husbands spend their time in drinking places. They sometimes spend their income on other women."
The project teaches men and women about gender equality, the division of labour, sharing farming proceeds and distribution of income. "After lengthy discussions on the division of labour within our households, my husband and other male farmers realized the negative impact and vowed to change," a proud Elizabeth adds. "We now share responsibilities in farming and make collective decisions on what to do with the proceeds. Before joining the forum, our income was 3500 Malawian kwacha (£14) per month but since we started working together it has increased to K12,200 (£47)."
Beata Nzayisenga, Rwanda
Beata Nzayisenga, 39, is a farmer trainee, who learned through a programme run by Tearfund how to set up a kitchen garden at her own home. Before the project, she recalls: "We used to eat vegetables only during rainy season and buy vegetables at higher price in dry season, but now we have it all the seasons – no matter dry or rainy – grown at our home."
The programme in which she is participating is funded by the Scottish Government grant to Ending Poverty One Village At a Time, and its activities focus on improving food security through improved agricultural techniques, conserving the environment by constructing terraces, water and sanitation, aiding access to finance and reducing risk of disaster.
Environmental sustainability is one of the challenges faced by members of her community. As a result, Tearfund is training them how to set up nursery beds that will be used to reduce soil erosion and improve soil fertility. Nzayisenga says: "We can now save the money used for buying vegetables, and malnutrition among children is reducing steadily."