To assemble each of 12 slushee cups, remove cupcake liners from 2 cupcakes. Put 1 cupcake in bottom of drinking cup, and poke with fork. Spoon 1 tablespoon drink mix syrup over cupcake. Top with about 1 tablespoon frosting mixture. Put second cupcake on top of frosting in drinking cup, and poke with fork. Spoon 1 tablespoon drink mix syrup over cupcake. Pipe frosting over top of cupcake. Insert straw into cup. Sprinkle with edible glitter. Repeat to use remaining cupcakes. Refrigerate at least 1 hour before serving. Store any leftover cupcakes in refrigerator.