Chocolate Cream Pie

Heat the oven to 350°F. Have a 9-inch pie pan ready. To make the crust: Place the hazelnuts on a rimmed baking sheet. Toast in the oven until the nuts are lightly browned and release a nutty fragrance, about 15 minutes. Remove the hazelnuts from the oven and set aside to cool. When nuts are completely cooled, use your hands to rub off as many of the skins as you can. Don't worry if you can't remove all of the skin; a little bit is OK.
In the bowl of a food processor fitted with a steel blade, process the hazelnuts, sugar, and cocoa powder until fine. Place in a bowl and add the melted butter, stirring to combine. Press the mixture onto the bottom and up the sides of the pie pan. Bake the crust until slightly crisp, about 25 minutes. Remove from the oven and cool on a wire rack.
In a medium, heavy saucepan, whisk together the sugar, cornstarch, salt, and egg yolks until well combined, then add the milk in a steady stream, whisking continuously. Bring to a boil over medium heat, whisking, then decrease the heat and simmer, whisking, for 1 minute more (the filling will be thick).
Force the filling through a fine-mesh sieve into a bowl, then whisk in the chocolate, butter, and vanilla. Cover the surface of the filling with a buttered round of waxed paper and cool completely, about 2 hours.
Spoon the filling into the crust. Chill the pie, loosely covered with plastic wrap, for at least 6 hours. Top with a thick layer of whipped cream.
Use a serrated knife to cut the pie into slices and use a cake server to gently place the slices on plates to serve. Note that the crust will be a little crumbly, so go easy.