Preheat the oven to 350℉. Spray two 8-inch cake pans with baking nonstick spray. Line with two parchment paper circles and spray again.
In a medium bowl, sift the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
3 1/4 cups All-purpose flour, 1/4 cup Cornstarch, 2 tsp Baking powder, 1/2 tsp Baking soda, 1 tsp Salt
Using a mixer in a large bowl, beat the butter, oil, and sugar on high speed for 2 minutes. It will be white in color and fluffy.
1/2 cup Unsalted butter, 1/2 cup Oil, 1 3/4 cups White granulated sugar
Add in the vanilla, egg whites, and sour cream and mix on medium speed until combined.
1 TBSP Vanilla paste, 5 Egg whites, 1 cup Sour cream
Add in the dry ingredients and buttermilk and mix on low speed until just combined. Take off the mixer and scrape the bowl to make sure it all mixed together.
1 cup Buttermilk
Divide the batter evenly into the two cake pans. Bake for 35-40 minutes. Bake until a toothpick inserted comes out clean.
Let the cakes sit in the hot pans for 10 minutes before transferring to a cooling rack. Cool cakes completely before decorating.