1. Place chocolate in small microwave-safe bowl. Microwave at MEDIUM (50%) 1 to 1-1/2 minutes or until chocolate is melted and smooth when stirred.
2. Beat butter, sugar, cream cheese and milk in medium bowl until well blended and smooth; fold in melted chocolate.
3. Fold in 2 cups whipped topping; spoon into crust. Cover; refrigerate until firm, about 3 hours. Garnish with additional whipped topping, if desired. 6 to 8 servings.