Whisk flour, sugar, 3/4 cup cocoa, baking powder, baking soda, and salt in a large bowl. Add buttermilk, oil, eggs, and 2 tsp vanilla, beat 2 minutes until smooth. Stir in hot coffee just until combined.

Pour batter into two greased and parchment-lined 8-inch round pans. Bake at 350°F (175°C) for 30 to 35 minutes, until a toothpick inserted in the center comes out with a few moist crumbs. Cool in pans 10 minutes, then turn out onto a rack to cool completely.

Carve a V-notch from the top of one cooled layer to form the bottom heart point. Cut the second layer in half vertically. Assemble pieces on a serving plate with a dab of frosting as glue, trim to shape, and chill 15 minutes.

Beat butter until creamy, then sift in powdered sugar and 1/2 cup cocoa. Add heavy cream, 2 tsp vanilla, and salt. Beat until fluffy. Crumb coat the cake, chill 20 minutes, frost fully, and serve.