Preheat oven to 450°. Line pastry shell with a double thickness of heavy-duty foil. Bake 8-10 minutes. Remove foil. Cool on a wire rack.
Meanwhile, in a heavy saucepan, combine marshmallow creme, chocolate chips, butter and unsweetened chocolate.
Cook and stir over low heat until chocolate is melted and mixture is smooth. Cool. Fold in whipped cream then pour mixture into crust.
Topping: in a large bowl, beat cream until it begins to thicken. Add powdered sugar; beat until stiff peaks form. Spread over filling.
Refrigerate at least 3 hours before serving. Garnish with chocolate shavings if desired.
