Preheat oven to 350°F (175°C) and grease two 8-inch round cake pans with butter and cocoa powder.

In a large mixing bowl, sift together the flour, black cocoa powder, sugar, baking soda, baking powder, and salt.

Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients and mix on medium speed until well combined.

Reduce mixer speed to low and slowly pour in the hot coffee, stirring until the batter is smooth and thin.

Divide the batter evenly between the prepared pans and bake for 30 to 35 minutes, or until a cake tester inserted into the center comes out clean.

Allow the cakes to cool in the pans for 15 minutes before inverting them onto wire racks to cool completely.

For the ganache, heat heavy cream in a small saucepan until it reaches a simmer, then pour over the chopped bittersweet chocolate.

Let the chocolate and cream sit for 5 minutes, then whisk gently starting from the center until a glossy, dark glaze forms.

Place one cake layer on a serving plate, spread a layer of ganache, top with the second cake layer, and frost the top and sides with the remaining ganache.