I've done a very similar cake but used the tube decorators icing for squeezed ketchup, mustard and mayo dripping out the sides. Also, a few sprinkled rice krispies make very convincing sesame seeds. Lastly, pound cake sliced like fries and lightly toasted make a great side of fries with "ketchup" squeezed on top.
I made this for my son's birthday and it was ADORABLE! I made just a few changes. For the lettuce we used green gum drops flattened with a rolling pin, which gave a ruffled edge to make the lettuce; omitted "tomatoes" and used red food coloring in the frosting for ketchup; instead of frosting the "bun", we left it unfrosted since the baked cake itself looked so "bun-like" and applied the chocolate frosting between the cake and brownie layers to get our frosting fix. We also rolled out some white gumdrops, then cut them into strips to make "onions". Thank you so much! This was such a joy to make and to give!