1Spray bottom and sides of 13x9-inch (3-quart) baking dish or pan with cooking spray. Set aside.
2In Dutch oven, heat marshmallow creme and butter over medium heat about 5 minutes, stirring constantly. Remove from heat.
3Add cereal; mix well.
4Spread mixture in baking dish. Using waxed paper or spatula sprayed with cooking spray, firmly press mixture evenly into baking dish. Cool before cutting. For bars, cut into 4 rows by 3 rows.