HEAT all but 3 squares of the chocolate with quarter cup milk in large microwave bowl at HIGH 1.5 to 2 min. Stir until completely melted or, heat in heavy saucepan over very low heat. BEAT in cream cheese, sugar and remaining milk. Gently stir in whipped topping until smooth. Spoon into crust. Freeze until firm. REMOVE from freezer. Let stand 30 min. at room temp. until you can cut easily. MELT remaining chocolate about 15 seconds; drizzle over pie.