Heat the oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners.

Mix the graham-cracker crumbs, melted butter, sugar, and pinch of salt. Place about 1 tablespoon in each liner, press it firmly but not excessively, and bake for 5 minutes.

Whisk the flour, cocoa powder, sugar, baking soda, baking powder, and salt in one bowl. In another bowl, whisk the egg, buttermilk, oil, and vanilla, then add the wet mixture to the dry ingredients and mix just until combined.

Stir in the hot coffee or hot water; the batter will be thin, which is normal. Divide it among the liners, filling each about two-thirds full.

Bake for 18–22 minutes, until the tops spring back lightly and a toothpick comes out with a few moist crumbs. Cool in the pan for about 5 minutes, then transfer to a rack and cool completely.

Combine the egg whites, sugar, cream of tartar, and salt in a heatproof bowl over barely simmering water. Whisk until the sugar dissolves and the mixture reaches 160°F (71°C).

Beat the warm egg-white mixture on high for 5–8 minutes, until thick, glossy, and stiff-peaked. Beat in the vanilla, then pipe or mound the frosting onto the cooled cupcakes.

Toast the frosting with a kitchen torch while keeping the flame moving. Finish with graham crumbs, chocolate pieces, melted chocolate, or one small marshmallow per cupcake if desired.