Using a box of yellow cake mix, bake 2 dozen cupcakes according to package directions. Don't use liners; just coat the baking trays with nonstick cooking spray. Remove cupcakes and let cool.
Repeat with a box of brownie mix to bake 2 dozen "burgers," then remove and let cool.
Divide one can of prepared white frosting evenly into three small bowls. Add green food coloring to one bowl, red food coloring to the other, and yellow food coloring to the third. Mix well.
Place each color in a plastic bag, secure end with rubber band, and cut about 1/8" off one of the bottom corners.
Slice cupcake "buns" in half horizontally. Place a brownie burger on the bottom half and top with green, red and yellow icing, making wavy lines along the rim of each burger to look like lettuce, ketchup and mustard.
Add top half of cupcake bun and sprinkle with sesame seeds.