A hamburger cake is as decadent, if not more, than the real burger you fashion it after. It combines yellow cake (as the bun) and brownies (as the burger, although you could also choose to use chocolate cake) with a variety of fruits or candies as the toppings.
Bake two yellow cakes as well as the brownie layer in round cake pans. Use a mix of confectioners sugar and water as a paste to adhere the brownie layer to the bottom bun and the condiments to the brownie. Choose strawberries and kiwis as tomatoes and lettuce, and dye whipped topping yellow and red for your mustard and ketchup.
Here's the tricky part -- the top bun. Be gentle, and consider placing the bun on top just before serving!