In a bowl, combine sugar, flour, cocoa powder, baking soda, baking powder, and salt. Stir with a whisk until evenly mixed.
Add vanilla, egg, buttermilk, and oil. Stir until combined.
Stir in the hot coffee quickly to ensure it is evenly distributed. The batter will be thin.
Pour the batter over the baked crust and bake for 20 minutes at 350°F or until a toothpick comes out clean.