Place the egg yolks in a 2 cup measuring cup or medium bowl and lightly whisk. Set aside.

In a medium saucepan, whisk together the sugar, flour, cocoa, and salt. Add the milk and whisk until smooth. Cook over medium heat, stirring constantly until the mixture begins to thicken. Pour half of the hot mixture in with the egg yolks while whisking constantly with a fork, then transfer the mixture back into the pan. Continue to cook for 1 minute over medium heat while stirring constantly. Remove from heat and whisk in the butter and vanilla. Pour the filling into the baked pie shell.

Allow the pie filling to cool for about 30 minutes at room temperature. Refrigerate it for 1 hour, then cover the filling with a piece of parchment paper. This prevents a skin from forming on the top. Don’t use plastic wrap like many recipes instruct you to do. When you remove the plastic wrap, the top layer of the filling will stick to the plastic. The parchment paper works perfect. Refrigerate the pie for 2 hours or overnight for best slicing.

Add the whipping cream to a large mixing bowl. Using the whisk attachment, (or use a hand mixer), whisk the cream on medium speed until it’s just starting to slightly thicken. Add the sugar and vanilla and whisk on high speed until the whipping cream forms soft to medium peaks. Serve immediately or refrigerate for up to 2 days.