In a medium saucepan, whisk together sugar, cornstarch and salt.
Add egg yolks, milk, and heavy cream and transfer to stovetop over medium heat and whisk ingredients together.
Whisk ingredients frequently until ingredients begin to thicken (this will take several minutes, do not crank up the heat or your filling may not thicken properly).
Once mixture has thickened² (it should be like a pudding consistency) whisk constantly, the mixture will come to a boil and continue to whisk for 60 seconds before removing from heat (still whisking constantly).
Immediately add chocolate, butter, and vanilla extract.
Whisk until ingredients are smooth and chocolate and butter are completely melted.
Pour your pie filling through a fine mesh sieve into a medium-sized heatproof bowl. Allow to cool for 15 minutes, stirring occasionally to prevent skin from forming.
Pour filling over prepared pie crust and cover surface with clear wrap, making sure that the clear wrap is pressed against the surface of the chocolate to prevent a skin from forming.
Refrigerate at least 6 hours to allow chocolate pie to firm up and set.
Homemade Whipped Cream
Prepare Whipped Cream shortly before serving pie.
Combine ¾ cup heavy cream, powdered sugar, and vanilla extract in a clean medium-sized bowl.
Use an electric mixer to stir on low-speed. Gradually increase speed to high and beat until stiff peaks form.
Pipe (I used a Wilton 2D tip) or dollop whipped cream topping over pie just before serving.
¹a 60 or 70% cacao bar works great, I often use Ghirardelli which is available right next to the cocoa powder in the baking section.²It is important that your mixture thickens and comes to a boil, otherwise it will not set properly.