Microwave butter in large microwaveable bowl on HIGH 25 sec. or until melted. Stir in dry jelly powder. Microwave 15 sec.; stir. Add marshmallows; toss to evenly coat. Microwave 1-1/2 min. or until marshmallows are completely melted and mixture is well blended, stirring after 45 sec. Add cereal; mix well. Press onto bottom of 13x9-inch pan sprayed with cooking spray; cool.