Preheat oven to 325°F and line a cupcake pan with cupcake liners.

In a small bowl, mix graham cracker crumbs, melted butter, and sugar until combined.

Press about 1 1/2 tablespoons of the crust mixture into the bottom of each cupcake liner.

Bake the crust for 7 minutes, then remove from the oven and allow it to cool.

In a large mixing bowl, whisk together flour, sugar, cocoa powder, baking soda, and salt.

Add the egg, buttermilk, and vegetable oil to the dry ingredients. Mix until smooth.

Stir vanilla into the boiling water, then slowly add the water to the batter while mixing.

Fill cupcake liners slightly more than halfway with batter.

Bake the cupcakes at 300°F for 17–19 minutes or until a toothpick inserted comes out clean.

Remove cupcakes from the oven and allow them to cool in the pan for 5 minutes before transferring to a cooling rack.

To make the marshmallow frosting, combine egg whites, sugar, cream of tartar, and vanilla in a heatproof mixing bowl.

Place the bowl over a saucepan of simmering water (double boiler method).

Whisk continuously until the sugar dissolves and the mixture reaches 120–140°F, about 4–6 minutes.

Remove from heat and beat with a mixer, starting on low speed and increasing to high.

Beat until stiff, glossy peaks form, about 5–7 minutes.

Pipe the marshmallow frosting onto the cooled cupcakes.

Drizzle with chocolate sauce and sprinkle with graham cracker crumbs before serving.