Whisk the flour, cocoa powder, baking soda, baking powder, and salt together in a bowl. Combine the sugar, buttermilk, oil, eggs, and vanilla in another large bowl. Add the dry ingredients to the wet ingredients gradually, stopping to scrape the bottom and sides of the bowl. Mix in the hot water. The batter will be very thin.
2 cups all-purpose flour, 3/4 cup unsweetened cocoa powder, 2 teaspoons baking soda, 1 teaspoon baking powder, 1 teaspoon salt, 1 3/4 cups granulated sugar, 1 cup buttermilk, 1/2 cup oil, 2 large eggs, 2 teaspoons vanilla extract, 1 cup very hot water