Pour melted chocolate into bottom of baked pastry shell, spreading evenly. Chill until chocolate hardens.
Combine 2/3 cup sugar, cornstarch, cocoa, and salt in a heavy saucepan; stir well. Stir together egg yolks, milk, and half-and-half; gradually stir into sugar mixture. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil 3 minutes, stirring constantly. Remove from heat; stir in butter, chopped chocolate, vanilla, and, if desired, bourbon.
Pour filling into a bowl; place plastic wrap directly over surface of filling, and cool 30 minutes. Pour filling into pastry shell; cover tightly with plastic wrap, and chill at least 6 hours.
Beat whipping cream at medium speed with an electric mixer until foamy; gradually add powdered sugar, beating until soft peaks form. Spread whipped cream over pie. Chill. Garnish, if desired.
Note: Find dark chocolate bars on the candy aisle at your local grocery store. Other types of chocolate used in this chapter can be found on the baking aisle.