MICROWAVE butter in large microwaveable bowl on HIGH 25 sec. or until melted. Stir in dry gelatin mix. 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 popcorn; stir gently until evenly coated.
SHAPE into 12 (2-inch) balls with greased or moistened hands.