Prepare the cake according to the recipe instructions. Bake the cake in an 8-inch round cake pan.

Allow the cake to cool for 10 minutes before turning it out onto a wire rack to cool completely.

In a small bowl, whisk the cocoa powder into the warm water until it dissolves completely. Set it aside.

In a small, heat-safe bowl, place the chopped chocolate and chopped butter, and place over a small saucepan of simmering water, making sure the bowl does not touch the water.

Melt the chocolate and butter over the simmering water, stirring occasionally, until smooth. Alternatively, you can melt them together in the microwave in short, 30-second bursts, stirring well in between.

Remove the melted chocolate and butter, and whisk the cocoa powder dissolved in water into it until smooth. Set the bowl aside.

In the bowl of a stand mixer fitted with the whisk attachment, or in a large bowl with a hand mixer, whip the heavy whipping cream on medium-high speed until soft peaks form. Add the confectioners’ sugar, and whip again until stiff, shiny peaks form.

Pour the chocolate mixture into the bowl of whipped cream slowly, carefully folding the ingredients together and taking care not to deflate the whipped cream more than necessary. Fold until no white streaks remain.

You can either use a 4-inch acetate collar, or one fashioned out of aluminum foil, with the inside sprayed with cooking oil to prevent the mousse from sticking to it.

First, cover the base of the springform pan tightly with plastic wrap, and place the inverted cake on top of the base. Trim the edges of the cake with a sharp knife if necessary to fit it inside the lip of the base of the pan.

Fit your cake collar around the cake (or makeshift cake collar), then clip the sides of the springform pan to the base to enclose the cake and collar.

Spoon the chocolate mousse onto the enclosed cake, and smooth the top.

Refrigerate the cake until the chocolate mousse is set (about 2 hours). Slice and serve.