LINE 2-1/2- to 3-qt. bowl with foil, extending foil about 2 inches beyond edge of bowl. Lightly grease foil; set aside.
PLACE butter in 4-qt. microwaveable bowl sprayed with cooking spray. Microwave on HIGH 45 sec. or until butter is melted. Add marshmallows; stir to coat. Microwave 1-1/2 min. or until marshmallows are completely melted and mixture is well blended, stirring after 45 sec. Microwave an additional 4 min., stirring after 2 min. Add cereal; mix well.
PLACE cereal mixture in foil-lined bowl; press firmly onto bottom and up side of bowl to form thick shell. Let stand in cool dry place at least 4 hours or overnight until firm. Remove cereal shell from bowl, using foil handles. Carefully peel off foil. Fill shell as desired.