In medium microwave-safe bowl, heat chocolate and cream in microwave oven on high 1 to 1-1/2 minutes or just until chocolate is melted, stirring every 30 seconds. Stir in vanilla extract and cool 20 minutes.
In 2 batches, gently fold one (8-ounce) container whipped topping into chocolate mixture. Spoon chocolate mixture into piecrust. Cover with plastic wrap and refrigerate at least 2 hours or up to overnight.
To serve, carefully spread remaining 8-ounce container of whipped topping over chocolate. Sprinkle with chocolate curls, if desired. (To make chocolate curls, with a vegetable peeler, shave along side of a 1-ounce square of semi-sweet chocolate.)