1. Place 1/3 cup chocolate chips and 2 tablespoons milk in microwave-safe bowl. Microwave 30 seconds at MEDIUM (50%); stir. If necessary, microwave an additional 15 seconds at a time, stirring after each heating, until chips are melted and mixture is smooth when stirred. Spread on bottom of crust. Refrigerate while preparing next step.
2. Place marshmallows, remaining 2/3 cup chocolate chips and remaining 1/4 cup milk in small saucepan. Cook over medium heat, stirring constantly, until marshmallows are melted and mixture is well blended. Transfer to separate large bowl; cool completely.
3. Stir 2 cups whipped topping into cooled chocolate mixture; spread 2 cups mixture over chocolate in crust. Blend remaining whipped topping and remaining chocolate mixture; spread over surface of pie.
4. Cover; freeze several hours or until firm. Garnish as desired. Cover; freeze remaining pie. 6 to 8 servings.