Preheat the oven to 350℉. Spray a 8-inch cake pan with baking nonstick spray. Line the bottom with a parchment paper circle and spray again.
In a medium bowl, sift the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
1 3/4 cup All-purpose flour, 1 TBSP Cornstarch, 1 tsp Baking powder, 1/4 tsp Baking soda, 1/2 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.
1/4 cup Unsalted butter, 1/4 cup Oil, 1 cup White granulated sugar
Add in the egg whites, vanilla, and sour cream. Mix on medium speed until combined.
1 TBSP Pure vanilla extract, 3 Egg whites, 1/2 cup Sour cream
Add in the flour mixture and buttermilk. Mix on low speed until just combined. Take off the mixer and use a rubber spatula to make sure the batter is all mixed together.
1/2 cup Buttermilk
Pour the cake batter in the cake pan and spread evenly.
Bake for 38-43 minutes. Bake until a toothpick inserted comes out clean.
Let the cake sit in the hot pan for 10 minutes before turning out on a cooling rack. Cool the cake completely before decorating.