Preheat oven to 350°F (180°C). Melt the 4 oz chopped dark chocolate over simmering water until smooth. Butter two 9-inch round cake pans, line with parchment, and butter the paper.

Whisk together the flour, cocoa, sugar, baking soda, baking powder, and salt. In another bowl, combine the buttermilk, oil, eggs, and vanilla. Add the wet mixture and melted chocolate to the dry ingredients and whisk just until combined. Stir in the hot coffee.

Divide the batter between the pans and bake for 25 to 30 minutes, until a toothpick comes out with moist crumbs. Cool in the pans for 10 minutes, then turn out and cool completely.

Place the 8 oz chopped chocolate for the ganache in a bowl. Heat the cream until just simmering, pour over the chocolate, let sit 2 minutes, then stir until glossy. Cool 15 minutes until thickened. Spread a third on one cake layer, top with the second layer, and pour the remaining ganache over the top. Slice and serve.