Position oven rack 1/3 up from the bottom of oven; preheat to 350°.
Add butter and chocolate to the top of a double boiler over warm water on low heat.
Cover until partially melted, then uncover and stir until completely melted and smooth.
Remove the top of the double boiler and set aside, uncovered, to cool slightly.
Meanwhile, in a large bowl beat (use electric mixer) the eggs well.
Except for the chocolate/butter mixture, add all the remaining ingredients; beat well.
Then beat in the chocolate and butter mixture.
Turn into the prepared, partially baked crust; the filling will come almost to the top of the crust.
Handle very carefully and place in the oven.
Bake for 50 minutes; do not bake any longer even if the filling appears soft.
Turn oven heat off; prop the oven door partially open, and let the pie stand in the oven until it is completely cool (the filling will puff up during baking and then will settle down to a thin layer that will crack while cooling).
Refrigerate for several hours.
Whipped cream: in a chilled bowl with chilled beaters, whip the cream with the remaining ingredients until it will just hold a shape.
Shortly before serving spread the cream in a thick layer over the top of the pie.