In a large mixing bowl, mix the flour, cocoa powder, cornstarch, baking powder, baking soda, and salt. Set aside.
1 cup All-purpose flour, 2 TBSP Cornstarch, 1/2 cup Unsweetened cocoa powder, 1 tsp Baking powder, 1/2 tsp Baking soda, 1/2 tsp Salt
In a medium bowl, mix the oil, sugar, eggs, vanilla, vinegar, sour cream, and buttermilk. Slowly add in the hot water.
1/2 cup Oil, 3/4 cup White granulated sugar, 2 Large eggs, 1 tsp Pure vanilla extract, 1 tsp White vinegar, 1/4 cup Sour cream, 1/2 cup Buttermilk, 1/2 cup Hot water
Pour the wet ingredients into the dry ingredients. Mix until completely combined and there are no flour clumps.
Pour 1/3rd of the chocolate cake batter in the pan. Spread so it is evenly across the bottom of the pan. Dollop 1/3rd of the cheesecake batter over the top of the cake batter. Don't use a knife to swirl!
Add another 1/3rd cake batter on top and another 1/3 cheesecake batter. Finish the rest of the cake batter and top with the remaining cheesecake batter. Use a spatula to make sure the top is smooth.
Bake for 45-55 minutes. Bake until the middle has a slight jiggle. Keep the cake in the pan and cool on a cooling rack completely.
Wrap the pan in foil and let it chill for 6 hours or overnight.