Preheat the oven to 350°F (175°C). Grease three 8-inch square cake pans. Line the bottoms with parchment paper and lightly grease the parchment. Position a rack in the center of the oven.

In a large bowl, whisk the flour, cocoa powder, granulated sugar, baking soda, baking powder, and salt until evenly combined and no large cocoa lumps remain.

Add the eggs, buttermilk, vegetable oil, and vanilla. Mix on low speed just until the dry ingredients are mostly incorporated. Do not overmix.

Slowly pour in the hot coffee while mixing on low speed. Mix only until the batter is smooth and evenly combined. The batter will be quite thin; this is normal.

Divide the batter evenly among the three prepared pans. For layers of equal height, weigh the filled pans or divide the batter by weight. Bake for 25–30 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs and no wet batter.

Let the cakes cool in their pans for 10–15 minutes. Carefully turn them onto wire racks, remove the parchment, and allow them to cool completely before frosting. Do not frost warm cake layers.

Beat the softened butter for 3–4 minutes until creamy. Add the cocoa powder and mix on low speed. Gradually add the powdered sugar, alternating with the heavy cream. Add the vanilla and salt. Beat until smooth and fluffy but still firm enough to hold its shape. If needed, add additional cream 1 teaspoon at a time.

Place a small dab of buttercream in the center of an 8- to 10-inch square cake board. Place the first cake layer on top. Spread an even layer of chocolate buttercream over it. Add the second layer and repeat. Place the third layer upside down so its flat bottom becomes the top of the cake.

Cover the entire cake with a thin layer of chocolate buttercream to trap loose crumbs. Check that the layers are straight and level. Refrigerate for 20–30 minutes, or until the crumb coat feels firm.

Apply a thicker final layer of chocolate buttercream to the top and all four sides. Use a tall straight cake scraper to smooth each side. Leave a small ridge of frosting above the top edges, then gently pull those ridges inward with an offset spatula to create defined square corners. Chill the frosted cake for at least 20 minutes before adding the ganache.

Place the finely chopped semisweet chocolate in a heatproof bowl. Heat the heavy cream just until steaming with small bubbles beginning to appear around the edge. Pour the cream over the chocolate and leave undisturbed for 2 minutes. Stir slowly from the center outward until completely smooth and glossy.

Allow the ganache to cool until slightly warm and thick enough to flow slowly. Test one drip on the back of the chilled cake. It should travel gradually down the side without reaching the cake board. If it runs too quickly, let the ganache cool longer. If it barely moves, warm it very briefly.

Create different-length ganache drips around all four top edges. Spread a thin layer of remaining ganache over the top without disturbing the buttercream corners. Let the ganache begin to set before adding heavy decorations.

Pipe several small chocolate buttercream rosettes across the top. Insert the tallest rectangular wafer bars toward the back. Add chocolate-covered wafer fingers, sandwich cookies, truffles, chocolate cups, chocolate balls, and broken chocolate squares. Keep taller pieces toward the back and smaller pieces toward the front to create a full but structured arrangement.

Arrange a smaller number of sandwich cookies, chocolate pieces, wafer fingers, chocolate balls, and buttercream rosettes around selected areas of the bottom edge. Leave some sections undecorated rather than creating a complete border.

Chill the decorated cake briefly if needed to firm the buttercream and ganache. For the best texture, let refrigera