On a lightly floured surface, roll pastry dough to a 1/8 inch thickness; transfer to pie plate; flute edges. Line pastry with pie weight of beans to pre cook pastry.
Bake at 400 for 20 minutes, remove pie weights (bean) and bake until golden brown, about 10 minutes.
In a heavy saucepan, mix sugar and cornstarch. Whisk in cream. Cook and stir over medium heat until thickened and bubbly. Remove from heat. In a small bowl whisk in a small amount of hot mixture into the egg yolks; return all to the pan, whisking constantly. Cook and stir over low heat 1 minute longer. Remove from the heat; stir in the chocolate, Cognac or brandy and vanilla stir until chocolate is melted.
Transfer pie filling into pastry crust; press plastic wrap onto surface of pie. Cool to room temp. Then refrigerate covered until cold, about 4 hours.
For topping, in a small bowl, beat cream until it begins to thicken. Add Cognac or brandy, confectioner's sugar; beat until soft peaks form. Spread over pie. Sprinkle with cocoa.