Spray 13x9-inch (3-quart) baking dish or pan with cooking spray.
In large microwavable bowl, microwave butter uncovered on High about 45 seconds or until melted. Add marshmallows; toss to coat. Microwave 1 to 1 1/2 minutes longer. Stir until marshmallows are completely melted and mixture is well blended.
Add cereal; mix well. Using waxed paper or spatula sprayed with cooking spray, press mixture evenly into baking dish. Cool 15 minutes. For bars, cut into 6 rows by 3 rows.