1. Place the cake in a large bowl, and crumble it roughly with your hands. Break it up until it's entirely in fine crumbs.
2. Once it's finely crumbled, add about 2/3 of the frosting to start with. The exact amount needed depends on how moist the cake is and on your personal preference, so we're going to start with a portion of the frosting and only add more if it's necessary.
