Bake the pie crust in a 9-inch pie pan according to the package or recipe directions. Set aside to cool.
Combine butter and chocolate chips in a large glass or metal bowl; set aside.
In a large saucepan, whisk together sugar, cornstarch, cocoa, and salt. Add the milk slowly and whisk until combined.
Bring the mixture to a simmer over medium heat, stirring constantly to prevent the milk from scorching.
Once the mixture has reached a simmer, simmer, stirring continuously, for 1 minute, until thickened.
Remove from the heat and pour ½ cup of the mixture into the egg yolks, quickly stirring to prevent the eggs from curdling.
Pour the egg yolk mixture into the custard mixture and stir to combine.
Return the pot to medium heat and bring the custard back to a simmer. Simmer for 1 minute, stirring constantly.
Remove from the heat and stir in the vanilla until smooth.
Place a strainer over the bowl of chocolate and butter and pour the hot custard mixture into the bowl, pressing it through the strainer to remove any lumps. Let sit for a couple of minutes, then stir until butter and chocolate are melted.
Pour the custard into a bowl, cover tightly with plastic wrap, and refrigerate until cooled to room temperature.
To assemble the pie, pour the cooled custard into the cooled pie crust. Chill for at least 4 hours or overnight.
Just before serving, beat the 1 cup of cream till soft peaks form. Beat in vanilla and powdered sugar. Spread over pie and garnish with chocolate curls if desired.