• Preheat oven to 350 degrees F. Have ready an ungreased 9-inch removable bottom tart pan.
• In a large bowl, mix together the graham crackers crumbs and sugar. Add melted butter and stir until evenly mixed, then press as tightly as you can into bottom and up the sides of the tart pan.
• Set the tart pan on a baking sheet and bake at 350F for 10 minutes. Let cool.
• Combine the sugar, salt and cornstarch in a medium size saucepan. Whisk in the cold milk. In a separate bowl, whisk together the egg yolks.
• Heat the milk mixture just until hot and steamy, then gradually whisk it into the egg yolks to temper. Add egg mixture back to the saucepan and whisk constantly until thickened and smooth and big bubbles break the surface. Whisk in the butter until melted, then remove from heat and stir in the chocolate. Lastly, stir in the vanilla.
• Scrape the filling into the crust and cover with a piece of greased plastic wrap while you make the topping. You can make it right away or leave the filling to cool and come back and do it later.

Marshmallow Topping

• Put the egg whites, sugar, and cream of tartar in the bowl of a stand mixer. Set the bowl over a saucepan of gently simmering water (double-boiler style), making sure the bottom of the bowl doesn’t touch the water.
• Whisk vigorously while holding the bowl steady with an oven mitt until the sugar is completely dissolved and the mixture feels smooth (not grainy) between your fingers. This should take about 5 minutes, and an instant thermometer should read 160°F.
• Transfer the bowl to the stand mixer fitted with the whisk attachment. Beat on high speed for about 5 minutes, or until the meringue is billowy, glossy, and holds stiff peaks. Towards the end of the beating time, add the vanilla.
• Spread the meringue over the cooled tart, then place it under the broiler for 30–60 seconds, watching constantly, just until the top is lightly toasted. Remove immediately since meringue can go from golden to burnt very quickly.
• Let the tart cool at room temperature, then chill for 3 hours or until cold. Remove from pan (or serve out of the pan if you wish) and cut into slices. Store in the refrigerator.

Method 2 -- Torch

• Another way to do this would be to prepare the crust and filling earlier, chill, add marshmallow topping and torch the crust. That method gives you a variety of temperatures because you'll have the cold crust and filling and the warm topping. I chose to just do everything ahead of time and serve the whole tart cold.