Start by adding sifted flour, baking powder, and salt into a medium-sized bowl. Blend well.
In a large bowl, beat together the softened butter and sugar until well combined.
Slowly beat in the egg whites.
Beat in the vanilla, sour cream, and milk. Do not overbeat.
Slowly add the dry ingredients into the wet ingredients until combined and blended. Be careful not to overbeat.
Bake in the oven for about 14-16 minutes. Check center of a cupcake for doneness.
Remove from oven, let cool for 10 minutes before moving to a wire rack to cool completely.
Let the cupcakes cool completely before frosting.
To Make Very Vanilla Frosting
Beat the softened butter until smooth.
Slowly add in the powdered sugar until it is all mixed into the butter. It will be clumpy.
Add in the vanilla and blend.
Add in 2 -3 tablespoon of milk or as needed to create the desired consistency for frosting. To pipe the cupcakes, you will need a stiffer frosting to hold shape.