Preheat oven to 350°F (175°C), grease two 8-inch round pans and line with parchment. Whisk flour, sugar, ¾ cup cocoa, baking powder, baking soda, and salt. Add eggs, buttermilk, oil, and 2 tsp vanilla and mix for 2 minutes. Slowly mix in hot coffee until batter is thin and smooth. Divide between pans and bake 30-35 minutes until a toothpick comes out with moist crumbs. Cool in pans 10 minutes, then turn out to cool completely.

Beat 1 ½ cups softened butter on high for 3-4 minutes until pale. On low, gradually add sifted powdered sugar, ¾ cup cocoa, ½ tsp salt, and 2 tsp vanilla. Drizzle in 4 tbsp cream, beat on high 2 minutes, adding more cream if needed for a spreadable consistency.

Place one cooled cake layer flat side up on a serving plate. Spread about 1 cup buttercream evenly over the top. Place second layer flat side down on top and press gently.

Spread a thin layer of buttercream over the entire cake as a crumb coat. Chill at least 30 minutes until firm.

Pile remaining buttercream on top of the cold cake. Smooth over the top and down the sides with a bench scraper until even. Chill another 15 minutes.

Place 6 oz chopped chocolate in a heatproof bowl. Heat ½ cup cream until bubbles appear, then pour over chocolate and let sit 2 minutes. Whisk until glossy. Cool 10 minutes, stirring occasionally, until it drips in heavy ribbons. Spoon ganache over the edge of the cold cake to create drips, then fill in the top. Let set 20 minutes before serving.