Instructions

In a medium saucepan, whisk together sugar, cornstarch and salt.
Add egg yolks, milk, and heavy cream and transfer to stovetop over medium-high heat and whisk ingredients together.
Whisk ingredients frequently on medium heat until ingredients begin to thicken (this will take several minutes).
Once mixture has thickened* (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 3/4 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 scoop whipped cream topping over pie just before serving.
Notes