PREPARE cake mix according to package directions using one 13 x 9-inch baking pan. Cool completely in pan. Invert cake onto serving platter. Frost cake, using a little more frosting than usual on top ("tombstones" will be pushed into frosting).
WRITE spooky words such as "RIP" or "BOO" on Crunch bars using tube icing. Let sit for a few minutes to dry.
PRESS Crunch bars into cake to create "tombstones". Sprinkle chopped Butterfinger bars next to tombstones to create "dirt". Creatively place gummy bugs over top of cake.