Spoon 3/4 cup of vanilla and dark-chocolate frosting into separate zip-top bags. Press out the excess air and seal the bags.
Place the remaining frosting into separate microwavable bowls. One bowl at a time, microwave the frosting, stirring every 10 seconds, until it is the texture of slightly whipped cream, 20 to 30 seconds.
Hold a cupcake by the foil liner and dip the top into the frosting up to the edge of the liner to create an even coating. Allow the excess frosting to drip off the cupcake back into the bowl before inverting. Invert and tap the bottom of the cupcake to smooth frosting. Repeat with 11 more cupcakes. Repeat process with the other bowl of frosting and the remaining cupcakes.
Snip a small (1/8-inch) corner from the bags with the frosting. Pipe bones and skeleton faces on the cupcakes.
Arrange the cupcakes on a serving platter and sprinkle the ground cookie crumbs as dirt. Makes 24 cupcakes.