1. Prepare the oven and pans.
Preheat the oven to 350°F (175°C). Lightly grease two 9-inch square cake pans. Line the bottoms with parchment paper and lightly grease the parchment.

2. Mix the dry ingredients.
Whisk the flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large mixing bowl. Break up any cocoa lumps.

3. Add the wet ingredients.
Add the eggs, buttermilk, vegetable oil, and vanilla. Mix on low speed just until combined. Scrape the bottom and sides of the bowl.

4. Add the coffee.
Slowly pour in the hot coffee while mixing on low. Continue only until the batter is smooth. The batter should be fairly thin.

5. Fill the pans.
Divide the batter evenly between the two prepared pans. For matching layers, weigh the filled pans or divide the batter by weight.

6. Bake.
Bake for 28–34 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs but no liquid batter.

7. Cool completely.
Leave the cakes in their pans for 10–15 minutes. Turn them onto wire racks, remove the parchment, and allow them to cool completely.

8. Make the chocolate buttercream.
Beat the softened butter for 2–3 minutes until creamy. Gradually add powdered sugar and cocoa powder on low speed.

9. Finish the frosting.
Add the cooled melted semisweet chocolate, heavy cream, vanilla, and salt. Beat until thick and fluffy. If necessary, add heavy cream 1 tablespoon at a time to reach a spreadable consistency.

10. Fill the cake.
Level the cakes only if they have pronounced domes. Place the first layer on a cake board or serving plate. Spread approximately ½ inch of chocolate frosting over it.

11. Stack and crumb-coat.
Place the second layer on top. Cover the cake with a thin crumb coat. Refrigerate for approximately 20 minutes.

12. Apply the final frosting.
Cover the top and four sides with the remaining chocolate buttercream. Use an offset spatula to create the rustic swirled texture shown in the finished cake.

13. Create the treasure topping.
Position the tallest chocolate shards and broken bars toward the back-center. Add medium pieces through the middle. Place truffles and small chocolate eggs toward the front. Leave areas of frosting visible.

14. Set before serving.
Chill for another 20–30 minutes if the frosting feels soft. For the best buttercream texture, let a refrigerated cake lose some of its chill before serving.