Use a large (3quart) glass casserole pan & 1 cupcake tin of 6. After cake cools, transfer upside down onto large piece of cardboard covered with tin foil. Cover top of cake with a thin layer of chocolate frosting.
Add cupcakes, cutting off rounded muffin top edges and placing upside down, resembling Lego Block pegs. Cover all in chocolate. A butter knife works great for spreading frosting. Write Happy Birthday on the tin foil.