Microwave butter in large microwavable bowl on High 30 seconds. Add marshmallows; mix to coat. Microwave on High 1 minute or until marshmallows are smooth when stirred, stirring after 30 seconds.

Stir in cereal. Mix lightly until well coated. Press evenly into lightly buttered 9-inch pie plate. Fill crust with ice cream; cover. Freeze about 2 hours or until firm. Let stand at room temperature about 10 minutes before serving. Garnish with additional cereal. Serve with Kraft Chocolate Topping, if desired. Store leftover pie in freezer.