Prepare brownie mix according to instructions and bake in a 13 x 9 in. cake pan.
Prepare vanilla cake mix according to instructions and bake in cupcake wrappers.
Once cooled, use a small circular cookie cutter to cut out the brownie “burgers”.
Remove cupcake wrappers from each cupcake and slice in half, forming top and bottom “buns”.
Put brownie burgers on top of bun bottoms.
Add ketchup frosting to a large plastic food storage bag and snip the corner to allow for piping.
Do the same for the mustard frosting.
Pipe ketchup on each burger and pipe mustard, too
Top each decorated burger with its top bun.