First, start with one of your prepared yellow cakes. This will be the bottom part of your bun.
Next, frost one of the chocolate cakes with chocolate frosting. You can use both of the chocolate cakes on the burger-cake if you want or you can frost the other chocolate cake and serve it separately. I did it separately because I thought a double-burger-cake would be a little too huge.
Place the chocolate cake on the "bottom bun".
Mix in some yellow/red food coloring into some of the fondant (about 3/4 c. worth) to make a nice orange cheesy color. Roll out the fondant really flat, cut into a large square, and place over the chocolate cake.
Mix some green food coloring into some of the fondant (maybe a cups worth) and roll it into a long roll (like a snake), flatten out and crinkle using a salad fork or anything you can think of to get that crinkle-y look. Circle it around the cake.
Color and roll out fondant to make the onions and tomatoes and place over the lettuce.
Separate the vanilla frosting into 2 bowls. Color one bright red and the other bright yellow (ketchup and mustard), put in separate plastic bags and snip off a corner so you can frost the cake.
Be generous with the "ketchup and mustard" and make it look nice and messy.
Carefully place the second yellow cake over the top and top with sesame seeds. (Don't worry, you won't taste them). Another option is to roll out some fondant and it like sesame seeds for the top.
Optional: Use our sugar cookie recipe (see link in notes below) to make some French fry shaped cookies to serve on the side. Sprinkle sugar on top before you bake them for the "salt" look.
Display the cake with the fries on the side and small bowls of red frosting for "ketchup" to dip your cookies.