To make the filling: Whisk the egg yolks, sugar, cornstarch, and salt together in a heatproof bowl until thick and pale in color. Set aside.

In a medium saucepan, bring the milk to a gentle boil. Whisk about ½ cup of the hot milk into the egg yolks to temper them, so they won’t curdle, then, continuing to whisk, add the remainder of the milk in a steady stream.

Set the bowl over a medium saucepan of simmering water (do not let the bottom of the bowl touch the water) and cook the egg-milk mixture, whisking constantly, until thick and glossy, 5 to 7 minutes.

Remove the bowl from the heat and whisk in the vanilla and melted chocolate until well combined. Let the filling mixture sit for 2 minutes to cool slightly, then whisk in the butter until the custard is smooth and silky. Place a piece of plastic wrap directly on top of the chocolate filling so that a skin does not form and let cool for 30 minutes, or until it reaches room temperature.
When the filling has cooled, pour it into the prepared crust. Cover the pie with plastic wrap and place it in the refrigerator to chill for at least 3 hours, or overnight.

To make the marshmallow topping: Put the egg whites, cream of tartar, and sugar in a large heatproof bowl, set the bowl over a medium saucepan of simmering water (do not let the bottom of the bowl touch the water), and whisk constantly until the sugar is completely dissolved and the egg whites are warm to the touch, 3 to 4 minutes. The mixture will appear pale and opaque. Rub a little of the mixture between your fingers to make sure the sugar is completely dissolved.

Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment (or leave in the same bowl if using a handheld mixture). Add the vanilla and, starting on low speed, whip the egg whites, gradually increasing the speed to high, until the marshmallow topping is glossy and forms stiff peaks, 5 to 7 minutes.

Fill a pastry bag fitted with a large plain tip with the marshmallow topping and pipe it over the entire surface of the pie, making sure it touches the crust all around. Then loosely pile the rest of the marshmallow topping on top and use a spatula to spread it in big, luscious swirls. (If you don’t have a pastry bag, you can do both steps using a spatula.) Toast the topping lightly with a kitchen blowtorch.

The pie is best served the same day, but it can be covered with plastic wrap and refrigerate for up to 3 days.

Graham Cracker Crust
Makes one 9-inch piecrust

2 cups graham cracker crumbs (16 crackers)
¼ cup packed light brown sugar
8 tablespoons (1 stick), unsalted butter, melted

Position a rack in the lower third of the oven and preheat the oven to 350?F.

In a medium bowl, blend together the graham cracker crust crumbs and brown sugar. Drizzle in the butter and mix with a fork until the crumbs are evenly moistened.

Press the mixture evenly over the bottom and about halfway up the sides of a 9-inch pie dish. Bake for 6 to 8 minutes, until lightly golden. Let cool completely before filling.