Preheat your oven to 400 degrees F. Roll your pie crust out onto an 18×13-inch baking sheet. I used a full batch of the pie crust above, which resulted in a slightly thicker crust. You could use half the batch and make a thinner crust. Place a piece of parchment paper over the pie dough and cover the top with pie weights or dried beans. Bake the crust for 20 to 30 minutes, or until it’s just slightly golden and flakey. Remove from the over and let the crust cool completely.
Add the sugar, 1 1/2 cups heavy cream, buttermilk, cornstarch and salt to a large saucepan over medium heat. Whisk until the sugar dissolves. Bring the mixture to a boil, stirring occasionally, until it thickens slightly, about 5 minutes. Reduce the mixture to a simmer and cook for another 5 minutes, stirring occasionally.
Place the egg yolks in a bowl and lightly beat them. Steam 1/2 cup of the warm milk mixture into the egg yolks while stirring constantly. Pour the egg yolk mixture back into the saucepan with the milk and heat over medium heat, stirring constantly. Stir until it is combined and very think, about 2 to 3 minutes. Remove the mixture from the heat and pour it into a bowl. Stir in the chocolate, butter and vanilla until it is melted and combined. Place a piece of plastic wrap directly on top of the chocolate pudding and stick it in the fridge for 30 minutes.
Add the remaining cold heavy cream to the bowl of your electric mixer and beat on medium high speed until peaks form. Add in the powdered sugar and beat until combined. Remove pudding from the fridge and fold 1/2 cup of the whipped cream into the chocolate. Stick the remaining whipped cream in the fridge. Spread the chocolate evenly over the pie crust. Refrigerate the pie for at least 4 hours before serving. Right before serving, spread the whipped cream on top and cover with some chocolate shavings or sprinkles.