Preheat the oven to 350°F (175°C). Grease and line two 8-inch square cake pans with parchment paper.

In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.

Add the eggs, buttermilk, vegetable oil, vanilla extract, and Nutella. Beat until smooth and well incorporated.

Gradually stir in the hot coffee or hot water until the batter becomes glossy and pourable.

Divide the batter evenly between the prepared pans and bake for 35-40 minutes, or until a skewer inserted into the center comes out clean.

Allow the cakes to cool in their pans for 10 minutes before transferring to a wire rack to cool completely.

Beat the butter until creamy. Gradually add the powdered sugar, cocoa powder, Nutella, vanilla, heavy cream, and salt. Continue beating until the buttercream is light, fluffy, and silky.

Level the cake layers if needed. Spread a generous layer of Nutella chocolate buttercream between each layer, then cover the entire cake with a smooth coat of buttercream. Chill for 20 minutes.

Heat the heavy cream until it just begins to simmer. Pour it over the chopped dark chocolate and butter. Let stand for 2 minutes before stirring until smooth and glossy. Cool slightly until thick but pourable.

Carefully spoon the ganache around the top edge of the chilled cake, allowing dramatic drips to cascade down the sides. Spread the remaining ganache across the top.

Decorate generously with Ferrero Rocher chocolates, assorted chocolate bars, chocolate wafer fingers, chocolate sandwich cookies, mini chocolate bars, chocolate hazelnut truffles, chocolate curls, chopped roasted hazelnuts, and chocolate sprinkles. Finish with a light drizzle of Nutella for an extra indulgent touch.

Refrigerate the cake for 20-30 minutes to set the decorations. Let it stand at room temperature for 15 minutes before serving for the richest flavor and creamiest texture.