Place on baking sheet, cover with wax paper.
Melt the white chocolate chips in microwave as bag directs. Spoon into a sturdy ziptop bag, snip tip off a corner. Using drawing under wax paper as a guide, pipe white chocolate skull on wax paper, leaving eyes and nose uncovered.
Move drawings, pipe 11 more skulls. Refrigerate to set.
Spoon 2 tablespoons of frosting into a small ziptop bag, set aside. Use the remaining frosting to frost top of cupcakes. Roll edges of cupcakes in jimmies.
Peel skulls off wax paper, place on cupcakes.
Snip tip off corner of bag with reserved frosting and pipe mouths.