Microwave butter in large microwavable bowl on HIGH 30 seconds. Add marshmallows; toss lightly. Microwave on HIGH 1 min. or until marshmallows are completely melted, stirring after 30 seconds. Add cereal; mix lightly to coat. Press firmly onto bottom and up side of lightly greased 9-in. pie plate to form crust.
Stir boiling water into gelatin in large bowl at least 2 min. until completely dissolved. Stir in cold water. Add whipped topping; stir with wire whisk until well blended. Refrigerate 15 to 20 min. or until mixture is very thick and will mound.
Spoon filling into crust; cover. Refrigerate 2 hours or until firm.