For the graham cracker crust: Preheat the oven to 350 degrees F. Line two standard 12-cup muffin tins with paper liners.
In a small bowl, stir together the graham cracker crumbs, butter and sugar until well combined. Press a heaping tablespoon of the crust mixture into the bottom of each muffin cup and bake until golden and just set, 5 to 7 minutes. Set aside to cool.
For the cupcakes: In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder and salt. Add the buttermilk, water and oil and whisk to combine. Add the eggs and vanilla extract and whisk until smooth. Divide the batter among the muffin cups, filling them about two-thirds full. Bake until a toothpick inserted into the center comes out with a few crumbs attached, 12 to 15 minutes.
Let cool in the tins on a wire rack for about 5 minutes, then remove the cupcakes from the tins and allow to cool completely.
For the marshmallow frosting: Combine the egg whites, sugar and cream of tartar in a heatproof mixing bowl set over a pan of simmering water. Whisk constantly until the sugar has dissolved and the egg whites are warm and smooth to the touch, 3 to 5 minutes. Transfer to the bowl of a stand mixer fitted with the whisk attachment and beat on low speed, gradually increasing to medium and then finally up to high. Beat until stiff, glossy peaks form, 7 to 10 minutes. Turn the mixer to low and whisk in the vanilla.
Spread the frosting on the cupcakes. With a handheld pastry torch, lightly brown the frosting. Decorate each cupcake with a graham cracker piece and a shard of chocolate.