1 baked and cooled 9' deep dish pie crust
2 tablespoons unsalted butter
1 1/3 cups semisweet chocolate, chopped
1 teaspoon vanilla extract
2/3 cup granulated sugar
3 tablespoons cornstarch
2 tablespoons unsweetened cocoa powder
1/8 teaspoon salt
3 large egg yolks
1 cup heavy cream, divided
2 cups milk
1 cup heavy cream
1/4 cup confectioners' sugar
1/2 teaspoon vanilla extract
Place the chopped chocolate, butter, and vanilla extract in a 2-quart mixing bowl; set aside. In a medium saucepan away from heat, whisk together the sugar, cornstarch, cocoa and salt. Whisk in 1/4 cup of cold heavy cream until the mixture is smooth, with no lumps. Repeat with another 1/4 cup of the cream. Whisk in the egg yolks. Place the saucepan over medium heat, and gradually whisk in the remaining cream and milk. Bring to a boil, whisking constantly as the mixture thickens; boil for 1 minute. Remove the pan from the heat and pour the mixture over the reserved chocolate and butter. Whisk until the chocolate is melted and the mixture is smooth. Place plastic wrap or buttered parchment paper on the surface to prevent a skin from forming, and chill thoroughly.
Place the heavy cream in a chilled mixing bowl. Whip until the whisk begins to leave tracks in the bowl. Add the sugar and vanilla and whip until the cream holds a medium peak.
Transfer the cooled filling to the cooled, baked pie crust. Level the top with the back of a spoon or an offset spatula. Spoon or pipe the whipped cream on top. Chill the pie until ready to serve.