Place the chopped chocolate, butter, and vanilla extract in a mixing bowl and set aside.
In a medium saucepan, whisk together the sugar, cornstarch, cocoa, and salt.
Whisk in 1/2 cup of cold heavy cream until the mixture is smooth. Repeat with another 1/2 cup of the cream. Whisk in the egg yolks.
Place the saucepan over medium heat, and gradually whisk in the whole milk.
Bring to a boil, whisking constantly. Let boil for 1 minute.
Remove the pan from the heat and pour the mixture over the chocolate and butter.
Whisk until the chocolate is melted and the mixture is silky smooth.
Place plastic wrap on surface of the custard to prevent a skin from forming, and chill for 3-4 hours.
Make pie crust and pre-bake according to directions here -- All-Butter Pie Crust Recipe. Let cool. Once cooled and chocolate custard has completely chilled and set-up, spread chocolate filling into pie crust.
In a large mixing bowl, whip heavy cream until soft peaks form. Fold in powdered sugar and continue to beat until peaks form. Spread or pipe over chocolate custard.