Preheat the oven to 350℉. Line three 8-inch cake pan with baking nonstick spray. Line the bottoms with parchment paper circles. Spray again.
In a large bowl, sift the flour, cornstarch, cocoa powder, baking powder, baking soda, and salt.
2 cups All-purpose flour, 1/4 cup Cornstarch, 1 cup Unsweetened cocoa powder, 2 tsp Baking powder, 1 tsp Baking soda, 1 tsp Salt
In a separate bowl, mix the oil, sugar, brown sugar, eggs, vanilla, vinegar, sour cream, and buttermilk. Use a whisk to mix it. Slowly whisk in the hot water.
1 cup Oil, 1 cup White granulated sugar, 3/4 cup Brown sugar, 4 Large eggs, 1 tsp Pure vanilla extract, 1 tsp White vinegar, 1 cup Buttermilk, 1 cup Hot water, 1/2 cup Sour cream
Pour the wet ingredients into the dry ingredients. Whisk until smooth and no flour lumps.
Pour the cake batter evenly into the three cake pans. Bake for 22-25 minutes. Bake until a toothpick inserted comes out clean.
Let the cakes sit in the hot pans for 10 minutes before transferring to cooling racks. Cool completely before decorating.