Use a 6″ cake cutter to cut two 6″ rounds out of the refrigerated chocolate cake. Place the ring in the center of a cardboard cake round, and line the inside of the ring with a cake collar (a 20″ strip should be enough).
Use the remaining cake scraps to create the bottom layer of cake. Break them up, and crumble them with your hands, pressing them tightly, especially into the bottom corners, to create an even, solid foundation for your cake.
Use a spoon or pastry brush to apply ⅓ of the coffee soak to the cake layer. Then apply ½ of the chocolate stout ganache, and use a small offset spatula to spread it all the way to the edges of the acetate.Top the ganache with ½ of the hazelnut crunch, again taking care to make sure it’s pressed up against the acetate. Finally, pipe ⅓ of the chocolate hazelnut frosting in a spiral over the crunch layer.
Carefully place the second cake layer on next. If one of the two cake rounds is messier or more uneven than the other, use that one.
Take the second 20″ acetate cake collar and insert it into the first one. Tuck it just under the top edge of the second cake layer so it stays in place.
Now repeat the layers: ⅓ of the coffee soak, the rest of the chocolate stout ganache, the rest of the hazelnut crunch, and pipe in another spiral of the chocolate hazelnut frosting.
Insert the final cake round. Place whichever side is flatter and smoother facing up. Then apply the rest of the coffee soak, and pipe a final spiral layer of chocolate hazelnut frosting. Use a mini offset spatula to smooth or create designs in the top layer of frosting.
Take the whole cake and put it in the freezer for at least 3-5 hours, or overnight. If you plan on keeping it in the freezer for longer than overnight, cover the top with saran wrap.
Before serving, remove cake from freezer and defrost for at least 3 hours. When ready to serve, slide the cake ring up to remove it and peel off the acetate sheets.