Preheat the oven to 350 degrees F.
Sift all the dry ingredients into a large bowl and set aside.
In a large bowl add the butter and the sugar. Using a hand-held electric mixer, combine until light and fluffy
Add the egg whites and vanilla and beat on medium speed for about 1 minute. Add about 1/3 of the flour mixture to the batter and beat until incorporated. Add about half of the buttermilk and continue to beat on medium speed until well mixed. Continue adding dry and wet ingredients, scraping the bowl and beating until incorporated after each addition. End with the dry ingredients. The batter will be thick and glossy. Spray muffin tins with nonstick cooking spray and bake for 18 to 20 minutes. Remove from the oven and cool.
Scoop out a small hole just in the middle of the top of the cupcake with a paring knife. Fill with fresh whipped cream and seasonal fruit. Garnish with a sprig of mint and serve.