1. Preheat oven to 425°. Fit piecrust into a 9-inch pie plate according to package directions; fold edges under, and crimp. Line piecrust with aluminum foil, and fill with pie weights or dried beans.
2. Bake at 425° for 8 minutes. Remove weights and foil, and bake 5 to 7 more minutes or until golden brown. Cool completely on a wire rack (about 15 minutes).
3. Meanwhile, microwave chocolate in a large microwave-safe bowl at HIGH 1 to 1 1/2 minutes or until melted and smooth, stirring at 30-second intervals. Whisk in milk until smooth; let stand 2 minutes. Fold half of whipped topping into chocolate mixture until combined; pour mixture into crust.
4. Cover and chill 4 to 8 hours. Spread remaining whipped topping over pie; garnish, if desired.