Preheat oven to 375ºF. Mix graham cracker crumbs, sugar and butter. Press mixture into a greased 9 inch glass deep dish pie plate and bake for about 6 minutes. Let cool.
Off heat, whisk the sugar, cornstarch, and salt together in a medium size (3 quart) saucepan.
Whisk in the milk and egg yolks. Place the mixture over medium heat, constantly whisking and scraping the sides of the pan until the mixture begins to bubble and thicken. Add the butter 1/2 tablespoon at a time. Add the chocolate a little at a time, giving each addition of chocolate the chance to melt into the filling before adding more. Keep the mixture over the heat until the chocolate is melted.
Remove from heat and let cool slightly, then stir in the vanilla. Pour the chocolate filling into the pie shell. Cover the pie with plastic wrap (or use a piece of wax paper or parchment) and let the filling cool while you prepare the marshmallow topping.
Combine 3 egg whites, 1/2 teaspoon cream of tartar, 1/4 teaspoon salt, 1 1/4 cups of sugar and 1/4 cup of water in the bowl of a stand mixer.
Put water in a large saucepan (I use the same one I used for the chocolate filling) and set the mixing bowl over it so it's not touching the water. Bring the water to a boil and carefully whisk the egg white mixture over the boiling water until it is warm to touch. It will froth up a bit at this point. When the mixture is warm, transfer, the bowl to the stand mixer base. Attach the whisk and beat for 7 minutes. The mixture will become white and fluffy. After 7 minutes, beat in the vanilla and beat for another minutes or until you have a big billowy mass of marshmallow.
Spread (or pipe) the marshmallow cream over the pie. Let cool completely and chill until ready to serve.
Before serving, you torch the whole pie or lightly torch individual pieces.