Preheat oven to 350 degrees. Line bottoms of three, 8 inch round cake pans with parchment paper. Butter and flour sides of pans (I use Baking PAM to do this).
Use mixer on medium speed to beat butter and sugar together until lightened and fluffy, about 2 minutes.
On low speed, beat in egg whites, one at a time. Scrape down sides of bowl with a spatula as needed. After egg whites are mixed in, mix in vanilla bean paste or extract.
In a small bowl, whisk together flour, cornstarch, salt, and baking powder.
Alternate beating in flour mixture and buttermilk, in about 3-4 additions each, starting and ended with flour mixture.
Fold in sour cream until well combined.
Divide batter evenly between three prepared pans. Bake until tops are a light golden color and a toothpick inserted into the center of the cake comes out with just a few crumbs, about 19-21 minutes.
Let cakes cool in pan for at least 5 minutes before removing. Allow cakes to cool completely before frosting.
When cakes are cooled, use serrated knife or cake leveler to level cakes.