1. Preheat oven to 325F. Line a 9x9-inch (or 8x8-inch) baking dish with parchment, leaving an overhang for easy lifting.
2. In a medium bowl whisk together the flour, baking soda, baking powder, salt, and cocoa powder.
3. In a large bowl whisk together the sugar and oil. Add the eggs, buttermilk, vanilla, and hot coffee. Whisk until smooth.
4. Add the dry ingredients to the wet ingredients in two additions, mixing until just combined. Batter will be thin.
5. Pour into the prepared pan. Bake for 35 to 45 mins or until a toothpick inserted in the center comes out with moist crumbs. (If using an 8x8 pan it may take longer to cook.) Let cool in the pan for 15 mins. Place a large plate over the pan, then carefully flip it over to invert the cake onto the plate. Remove the parchment paper from the cake and let cool completely before frosting.
6. Make the frosting: Beat the butter and cocoa powder until combined. Add the powdered sugar, salt, vanilla, and milk and beat until light and creamy. Beat in the melted chocolate until fully incorporated.
7. Spread the frosting over the cooled cake. If desired, use a vegetable peeler to shave dark chocolate over the top. Slice into squares and enjoy.
Store covered in the fridge for up to 4 days. Reheat individual slices in the microwave for 10-20 seconds, just to take the chill off.
