Place raspberry chips and milk in large microwave-safe bowl. Microwave on HIGH for 1 minute or until chips are melted and mixture is smooth when stirred. Add marshmallows; microwave on HIGH for 1 minute; stir. If necessary, microwave on HIGH an additional 30 seconds at a time, stirring after each heating, just until marshmallows are melted when stirred. Spread 1/2 cup chip mixture over bottom of crust. Cool remaining mixture to room temperature; fold in whipped topping. Spoon into crust. Cover; refrigerate 4 to 6 hours or until firm. Garnish with whipped topping, if desired.