Heat the oven to 350°F (175°C). Mix graham crumbs and crust butter, press into the base and sides of the pie plate, and bake 10 minutes.
Cool the crust. In a saucepan whisk sugar, cocoa, cornstarch, and salt. Gradually whisk in milk and egg yolks until completely smooth.
Cook over medium heat, whisking constantly across the base and corners, until the mixture thickens and reaches a gentle bubbling boil.
Continue whisking for 1 minute after bubbling begins. Remove from heat and stir in chopped chocolate, filling butter, and vanilla until smooth.
Strain into a bowl if any egg bits remain, then pour into the cooled crust. Smooth the top and press food-safe wrapping directly against the filling.
Cool about 30 minutes, then refrigerate at least 4 hours until fully set. Keep the pie level and leave the covering against the surface.
Whip cold heavy cream and powdered sugar to medium-stiff peaks. Remove the filling cover and spread or pipe the cream over the chilled pie.
Slice with a clean sharp knife, wiping frequently. Keep the pie refrigerated until serving and return remaining portions to the refrigerator promptly.
