Spray a 9x13-inch baking dish with non-stick cooking spray. In a saucepan, melt the butter and marshmallows over medium heat. Once melted add apple pie spice and stir until dissolved.
Remove from heat and add the Rice Krispies. Stir until coated.
Spoon the Rice Krispies mixture into the prepared pan. Press firmly with wax paper. Once cooled, use cookie cutters to cut the Rice Krispies Treats into desired shapes.
Decorate Rice Krispies treats with decorator frosting and assorted candies to make your own monster.