Crush 8-1/2 crackers to form fine crumbs; break remaining crackers into rectangles.
Place cracker crumbs in medium bowl.
Add butter and sugar; mix well. Press onto bottom of 9-inch springform pan.
Bake 12 min. or until lightly browned. Cool completely.
Meanwhile, microwave chocolate and COOL WHIP in microwaveable bowl on HIGH 2 to 2-1/2 min. or until chocolate is completely melted and mixture is blended, stirring after each minute.
Let stand 15 min.
Spread chocolate mixture over crust. Stand cracker rectangles, top-sides facing out, around edge of pan, pressing gently into chocolate mixture to secure. Freeze 20 min. or until filling is firm.
Cover with ice cream. Freeze 4 hours or until ice cream layer is very firm. When ready to serve dessert, reserve 2 cups marshmallows. Microwave remaining marshmallows and milk in large microwaveable bowl on HIGH 1 min. 15 sec. or until marshmallows are completely melted and mixture is well blended, stirring after 1 min. Cool 10 min.
Heat broiler.
Place dessert on baking sheet; top with melted marshmallow mixture and reserved marshmallows. Broil, 6 inches from heat, 1 min. or until marshmallows are golden brown.
