Preheat oven to 350°F (175°C).
In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Press mixture into the bottom and up the sides of a 9-inch pie dish.
Bake crust for 10 minutes. Let cool completely.
In a medium saucepan, whisk together sugar, cornstarch, cocoa powder, and salt. Gradually whisk in milk until smooth.
Cook over medium heat, whisking constantly, until mixture thickens and comes to a boil. Boil for 1 minute, then remove from heat.
Add chopped chocolate, butter, and vanilla extract to the mixture. Stir until chocolate and butter are melted and mixture is smooth.
Pour filling into cooled crust. Cover with plastic wrap, pressing it directly onto the surface of the filling to prevent a skin from forming. Refrigerate for at least 2 hours, or until set.
In a large bowl, whip heavy cream, sugar, and vanilla extract until stiff peaks form. Spread whipped cream over the chilled pie.
Serve and enjoy!