Microwave butter in large microwaveable bowl on HIGH 1 min. or until melted Stir in dry jelly powder. Microwave 15 sec.; stir. Add marshmallows; toss to evenly coat. Microwave 1 to 1-1/2 min. or until marshmallows are puffed; stir until marshmallows are completely melted and mixture is well blended.