To make the chocolate cake: Preheat the oven to 375° F. Line with parchment and grease an 8-inch springform pan or grease 6-8 individual baking dishes. Sprinkle an even thin layer of graham cracker crumbs onto the bottom of the pan/dishes. Set onto a baking sheet and set aside.
Melt the chocolate gently in a metal or glass bowl over simmering water, making sure the bottom of the bowl doesn't touch the water. Once melted, then beat in the butter a piece at a time and stir until smooth.
In a separate bowl, beat the yolks with 1/2-cup of the sugar until thick, pale, and ribbony (a hand whisk is fine for this step).
In yet another bowl or the bowl of your stand mixer, beat the whites to soft peaks. Scatter over the remaining 1/2-cup sugar and the pinch of salt, then beat to a stiff meringue.
Slowly whisk the chocolate mixture into the yolk mixture. Stir in a spoonful of beaten egg whites, then pour the chocolate mixture over the egg whites and gently fold together with a spatula until there are no streaks of white in the batter. Pour the batter into the prepared pan (or dishes) and bake - 45-50 minutes for an 8-inch springform or 15-25 minutes for smaller, individual dishes (check often!). Cakes are done when the top has formed a crust with deep cracks and the centre doesn't jiggle when moved.
Remove from the oven. If using springform pan, run a knife around the outside edge, then let sit until cool. It will sink down and the top will crack as it cools. For individual dishes, just leave in and serve from the dishes. For a springform, remove from pan once cooled.
To make the graham cracker crumbles: Preheat oven to 350° F. Butter a small baking pan.
In a small bowl, mix together the graham cracker crumbs, brown sugar and cinnamon. Combine the melted butter and vanilla, then stir in. Spread evenly onto the prepared pan. Sprinkle white sugar and salt over the top.
Bake in preheated oven for 5-8 minutes, or until lightly browned. Remove from baking sheet and let cool. Break up any large crumbles, if necessary. Store in an airtight container once cooled, if making ahead.
To make the Marshmallow Topping: Fill a medium saucepan with 1 inch of water and bring it to a simmer over medium heat.
Place the egg whites, sugar, cream of tartar, and salt in a clean metal or glass bowl, then place the bowl over the saucepan, making sure the bottom of the bowl is not touching the water. Heat the egg white mixture, whisking constantly, until the sugar has dissolved and the mixture is hot to the touch (about 120° F. on an instant-read thermometer), about 3-5 minutes.
Transfer the mixture to a stand mixer fitted with the whisk attachment or beat with a hand mixer in the same bowl. Turn the mixer to medium and whisk for 1 minute. Increase the speed to high and whisk until stiff, glossy peaks form, about 5 minutes more. Add the vanilla and whisk until just incorporated, about 1 minute. Use immediately or store in an airtight container at room temperature for up to 3 hours.
To assemble cakes: If desired, you can re-heat your cakes slightly in the microwave (20-30 seconds). Sprinkle a bit of the graham crumble in to the cavity area of the cake, then top with a dollop of the marshmallow topping. If desired, dip the end of a wire whisk slightly in to the topping and pull up quickly to create a few spikes of topping. (You could also pipe the marshmallow on with a piping bag, if you like). Using a kitchen torch, toast the marshmallow slightly. If you don't have a torch, you can place under a broiler briefly (not too close to the heat source) until it starts to colour. Top with a sprinkling of the graham cracker crumbles. Serve immediately.