MIX graham crumbs, butter and sugar; press firmly onto bottom and up side of 9-inch pie plate. Microwave cream in medium microwavable bowl on HIGH 1-1/2 minutes or until cream just comes to boil. Add chocolate; stir. Let stand 2 minutes; stir with wire whisk until chocolate is completely melted and mixture is well blended. Pour into crust. Refrigerate 30 minutes or until firm.
MEANWHILE, place 2 cups of the marshmallows and milk in large microwavable bowl. Microwave on HIGH 1-1/2 minutes or until marshmallows are completely melted, stirring after 1 minute. Refrigerate 15 minutes or until cooled.
ADD whipped topping; stir with wire whisk until well blended. Spread over chocolate layer in crust. Refrigerate 3 hours or until firm. Sprinkle with remaining 1/2 cup marshmallows just before serving. Store leftover pie in refrigerator.