Roll the pie dough into a 12" to 13" circle on a piece of plastic wrap, a silicone rolling mat, or in a pie bag that's been heavily sprinkled with gluten-free flour or flour blend. Invert the crust into the prepared pie pan, pressing it into the pan.
Line the bottom of the crust with pie weights. Or line with parchment, then add dry rice, or dried beans.
Bake the crust for 25 minutes. Remove the weights, and bake for an additional 10 to 15 minutes, until the crust is light golden brown; it's helpful to have a pie shield or some foil at the ready in case the edge of your crust begins to brown too quickly.
Remove the crust from the oven, and allow it to cool until you can touch it comfortably.
To make the filling: Place the butter, chocolate, and vanilla into a 2-quart bowl; set aside.
In a medium saucepan off the heat, whisk together the sugar, cornstarch, cocoa, and salt.
Whisk in 1/4 cup (57g) cold heavy cream until the mixture is smooth, with no lumps. Repeat with another 1/4 cup (57g) of the cream, then whisk in the egg yolks.
Over medium heat, gradually whisk in the remaining cream, and the milk.
Bring the mixture to a boil, whisking constantly as it thickens; boil for 1 minute.
Remove the pan from the heat and pour the mixture over the reserved chocolate and butter. Whisk until the chocolate is melted and the mixture is smooth.
Pour the filling through a strainer into a bowl, to remove any lumps.
Place plastic wrap or buttered parchment paper on the surface to prevent a skin from forming, and chill thoroughly.
To assemble the pie: Spoon the cooled filling into the cooled, baked pie crust. Level the top with the back of a spoon or an offset spatula.
Spoon or pipe whipped cream on top, if desired. Or top individual slices with whipped cream when serving.
Keep the pie chilled until ready to serve. For best slicing, refrigerate the pie overnight before serving.