Line two 13x9-inch pans with foil; lightly spray foil with cooking spray. Set aside. Microwave butter in 4-qt. microwaveable bowl on HIGH 45 sec. or until melted.
Add marshmallows; toss to coat. Microwave 1-1/2 min. or until marshmallows are completely melted and mixture is well blended, stirring after 45 sec.
Add cereal; mix well.
Press half of the cereal mixture firmly and evenly into each prepared pan. Cool 15 min.
Cut mixture in each pan into 16 rectangles, about 3x2-inches each.
Spread one rectangle with 1/4 cup of the ice cream; top with second rectangle to make sandwich. Coat edges with decorations as desired. Repeat to make a total of 16 sandwiches. Wrap individually in plastic wrap; freeze 3 hours or until firm.
Remove from freezer about 10 min. before serving to soften slightly.
