Preheat the oven to 325 F, and position a rack in the center of the oven. Lightly spray the bottom and sides of a 9-inch round tart pan with a removable bottom or a deep-dish pie pan with non-stick baking spray.
Combine the crushed graham crackers and salt, then add the melted butter and toss with a fork until moist. Press the crumbs firmly against the bottom and up the sides of the pan.
Bake the crust for 8-10 minutes, until toasted and fragrant, then set aside to cool.
Filling
In a large saucepan, whisk together the cocoa powder, sugar and corn starch.
In a bowl or liquid measuring cup, whisk together the milk and egg yolks. Slowly pour the milk and egg yolks into the saucepan, whisking together with the dry ingredients until smooth.
Over medium heat, cook the custard while whisking constantly, until it thickens and begins to boil, then continue cooking for one more minute. Depending on the heat, it might take 8-10 minutes to thicken.
Remove the custard from the heat and whisk in the butter, vanilla and chopped chocolate until smooth.
Pour the custard through a fine mesh strainer (to remove any bits of cooked egg) into a bowl. Cover the bowl with plastic wrap, resting right against the surface of the custard (this prevents a skin from forming on top), and let it cool in the refrigerator for 1 hour; it will still be warm, but no longer steaming hot.
Spread the custard into the baked crust, and chill in the refrigerator for about 2-3 hours, until set. If you like, sprinkle the top of the filling with a little more chopped chocolate or chocolate curls.
Whipped Cream
Before serving, whip the cold heavy whipping cream, powdered sugar and vanilla until thick; serve with the pie.
If you like a saltier crust, use the full 1 teaspoon (I use Kroger brand coarse Kosher salt, but Morton's is saltier, and you should use half the amount with Morton's). For a less salty flavor, just use 3/4 teaspoon. If using finer grain table salt, use 1/4 - 1/2 teaspoon.Leftover pie should be stored in the refrigerator, loosely covered, and is best eaten within 2-3 days.