Spray 9x13-inch pan with nonstick spray. Set aside. Place butter in large bowl. Microwave on HIGH 30 seconds or until melted. Add marshmallows; stir to coat well with butter. Microwave 1 minute; stir. Continue microwaving 30 to 45 seconds, just until completely melted. Stir in vanilla. Immediately add cereal; stir to mix well and completely coat cereal. Press cereal mixture evenly into prepared pan. Cool completely. Cut into 24 bars.