three 8-inch-round cake pans, 2 piping bags, 2 medium star tips, fondant smoother (optional), pastry brush
For the cake: Preheat the oven to 350 degrees F. Grease three 8-inch-round cake pans with butter and line with parchment paper.
Place the sugar and oil in the bowl of a stand mixer fitted with the whisk attachment and beat on medium speed until combined. Add the eggs, one by one, until completely incorporated. Add the vanilla extract and buttermilk and continue mixing until combined.
Sift the flour, cocoa powder, baking powder, baking soda and salt in a large bowl. Mix together the coffee and warm water. Alternate adding the dry ingredients and the coffee mixture into the bowl of the stand mixer, beginning and ending with the dry ingredients. Mix until you don’t see any lumps and a thin batter has formed.
Divide the batter evenly among the prepared pans and bake until a toothpick inserted in the middle of each cake comes out clean, 30 to 35 minutes. Transfer the pans to a wire rack and allow to cool for 10 minutes. Remove the cakes from the pans and set on the wire rack to cool completely, about 1 hour.
For the dark chocolate mousse: Place the dark chocolate in a large bowl and set aside. Pour 1 cup of the heavy cream into a small saucepan and bring to a boil over medium-high heat. Pour the cream over the chocolate, cover the bowl with a kitchen towel and let sit for 3 minutes.
Remove the towel and slowly whisk the ganache until it’s smooth and shiny. Set aside to cool to about room temperature.
Meanwhile, whip the remaining cup of heavy cream to stiff peaks in the bowl of a stand mixer fitted with the whisk attachment, or by hand. Using a rubber spatula, fold one-fourth of the whipped cream into the cooled ganache to lighten the consistency, then fold that mixture into the remaining whipped cream until almost completely combined with just a few streaks. Add the vanilla and salt and gently fold until combined into a light, fluffy mousse.
For the chocolate fudge buttercream frosting: Place the sugar, hot fudge, cocoa powder, butter, milk, vanilla extract and salt in the bowl of a stand mixer fitted with the paddle attachment and beat on low speed to combine. When the sugar is incorporated, increase the speed to medium and beat until the buttercream is smooth and satiny, 8 to 10 minutes.
For the chocolate royal icing: Place the confectioners’ sugar, cocoa powder, meringue powder and water in the bowl of a stand mixer fitted with the whisk attachment and whip on low speed to combine. Increase the speed to medium-high and whip until the icing holds stiff peaks, 5 to 8 minutes. If it seems too soft, add more confectioners’ sugar in 1 tablespoon increments. If it is too stiff, add a bit more water, drop by drop. Transfer to a piping bag fitted with a medium star tip.
For the white chocolate ganache: Place the white chocolate in a medium bowl and set aside. Pour the cream into a small saucepan and bring to a boil over medium-high heat. Pour the cream over the chocolate, cover the bowl with a kitchen towel and let sit for 3 minutes.
Remove the towel and slowly whisk the ganache until it’s smooth and shiny. Set aside to cool to about room temperature, then transfer to a piping bag fitted with a medium star tip.
To assemble, level the tops of the cakes with a serrated knife or cake leveler. Spread half of the mousse over the top of one cake layer. Stack the second layer and repeat. Add the third cake layer and frost the top and sides with a thin coat of the buttercream. Refrigerate until the frosting is firm, 20 to 30 minutes.
When the cake is ready, frost the top and sides with a thicker layer of the buttercream and refrigerate again until very firm, about 1 hour.
When the cake is almost ready, lightly dust your work area with cornstarch and roll the fondant into a circle about 20-inches wide and 1/8-inch thick. Drape the rolled fondant over the cake and, using a fondant smoother or your hands, lightly press and smooth it onto the cake, working your way from the top down the sides. Pop any air bubbles with a cake tester or toothpick and continue to smooth the surface.
Trim any excess fondant from the base of the cake with a pizza cutter. Using a pastry brush, lightly brush the bottom half of the cake with the piping gel, then cover it with chocolate sprinkles.
Pipe a scalloped border around the edge with the chocolate royal icing. Pipe little rosettes of the white chocolate ganache around the cake and press 1 hazelnut truffle onto each. Continue to pipe and decorate with the royal icing, white chocolate ganache and hazelnut truffles to your liking.
Never leave a child unattended in the kitchen. Limit the child to tasks that are safe and age-appropriate.