Preheat oven to 350°F (177°C).

Read my tips about making a perfect graham cracker crust before you get started. Using a food processor or blender, grind graham crackers into fine crumbs. (Alternatively, you can crush them in a plastic bag with a rolling pin.) In a medium bowl, mix graham cracker crumbs, melted butter, and granulated sugar until combined. (The mixture will be thick, coarse, and sandy.) Press crumbs evenly into each tartlet pan, making sure the crust is tight and compact. Pre-bake crusts for 10 minutes, then let cool.

Place chocolate and butter in a large bowl and set aside. In a small saucepan over medium-low heat, heat heavy cream. Whisking constantly, bring to a slow boil. Once it begins to boil, immediately remove from heat and pour over chocolate. Using a wooden spoon, slowly stir until chocolate and butter are completely melted and ganache is smooth.

Pour the filling evenly into each crust and place each tartlet pan in the refrigerator. Chill at least 2 hours (and up to 1–2 days) or until completely firm.

Place egg whites, sugar, and cream of tartar in the heatproof bowl of an electric mixer. Set bowl over a saucepan containing about 2 inches of simmering water (making sure the bowl isn’t touching the water below). Whisk constantly until sugar is dissolved and egg whites are warm to the touch, about 4 minutes. Transfer the mixing bowl to an electric mixer fitted with a whisk attachment. Beat on high speed for 4–5 minutes until soft, glossy peaks form. Add vanilla and beat on high until just combined.

Remove tartlets from the refrigerator. Spread marshmallow topping on each. Toast the topping with a kitchen torch* if desired. Serve immediately. Any leftover tarts keep well in the refrigerator, in an airtight container, for up to 3 days.