Spray 13x9-inch pan with cooking spray. In large microwavable bowl, microwave 1/4 cup butter uncovered on High 30 to 45 seconds or until melted. Add marshmallows; toss until coated. Microwave uncovered on High 1 minute 15 seconds to 1 minute 45 seconds, stirring every 30 seconds, until mixture can be stirred smooth. Stir in cereals. Press evenly in pan.