Bake pie crust as directed on package for unfilled one-crust pie; cool completely.
Melt chocolate as directed on package; cool slightly. Beat butter and sugar in large bowl with electric mixer on medium speed 3 min. or until light and fluffy.
Add melted chocolate and vanilla; mix well.
Add egg product. Beat on high speed 2 min. or until light and fluffy. Spoon into pie crust.
Refrigerate 3 hours or until chilled. Top with the whipped topping. Store leftover pie in refrigerator.