Preheat oven to 350F and spray three 8 inch cake pans with non-stick baking spray. Place a parchment circle on the bottom and spray again.
In a large mixing bowl, mix flour, cornstarch, baking powder, baking soda, salt, cocoa powder together.
2 cups All-purpose flour, 1 cup Dutch process cocoa powder, 1 tsp Baking soda, 2 tsp Baking powder, 1 tsp Salt, 1/4 cup Cornstarch
In a separate bowl, mix oil, sugar, buttermilk, vanilla, and eggs. Heat the hot water on the stove, then very slowly pour the hot water into the wet ingredients while mixing.
1 cup Buttermilk, 1 cup Oil, 4 Large eggs, 1 tsp Pure vanilla extract, 1 cup Hot Water, 1 3/4 cups White granulated sugar
Pour the wet ingredients into the dry. Whisk together until just combined. Don't over mix.
Evenly spread batter into the three cake pans. Bake for 20-24 minutes. Bake until a toothpick inserted comes out clean.
Let the cakes sit in the hot pan for 10 minutes. Then, transfer to a cooling rack and cool completely.