Preheat oven to 350°F (176°C) and prepare an 8-inch cake pan with parchment paper.
Cream softened butter and sugar until light and fluffy.
Add sour cream and vanilla, mix well.
Add egg whites one at a time, mixing after each.
Combine dry ingredients in a separate bowl.
Alternate adding dry ingredients and the milk mixture to the batter.
Bake for 19-21 minutes, cool, and prepare for layering.