In small microwave-safe bowl, place chocolate chips. Microwave at High (100%) 1 to 1 1/2 minutes or until chips are melted and mixture is smooth when stirred. Set aside to cool. In large mixer bowl, beat cream cheese, sugar, and butter until creamy; on low speed, blend in melted chocolate. Fold in whipped topping until blended; spoon into crust. Cover; refrigerate until firm.