Preheat the oven to 350 degrees.
In a small bowl, whisk together the flour, baking powder and salt. Set aside.
In another bowl, whisk together the milk, egg whites, sour cream and vanilla. Set aside.
In a large bowl, beat the butter and sugar together for about 2 minutes, until light and fluffy.
Alternate adding a small amount of the flour mixture and the sour cream mixture, starting and ending with the flour mixture.
Once all ingredients are thoroughly combined, divide evenly among a lined cupcake tin.
Place in the oven and bake for 20 minutes, or until a toothpick comes out cleanly.
Place on a cooling rack and allow to completely cool before frosting.