Preheat oven to 400°F. Arrange pepperoni pieces on pizza to resemble bats and mushroom slices on pizza to resemble pumpkins.
Bake pizza as directed on package. Meanwhile, use pumpkin-, bat- or witch's hat-shaped cookie cutters to cut shapes from Singles.
Remove pizza from oven. Immediately top with Singles cutouts. Let stand 5 minutes to allow Singles to melt.