Spray a 9 x 13-inch baking pan with oil; set aside.
In a large glass bowl, microwave the butter and the marshmallows on high power for 2 minutes. Stir well and microwave on high 1 minute longer.
Add cereal and stir until well-coated. Spray a sheet of waxed paper with canola oil. Use it to press warm cereal mixture into the prepared pan. (If using rice Krispies, immediately top with sprinkles.) Let guests use 4-inch star cookie cutters to make 12 pieces.