Preheat oven to 350 °F (175 °C). Grease and line three 8-inch round cake pans with parchment paper.

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

In another bowl, combine buttermilk, oil, eggs, and vanilla. Add to the dry ingredients and mix until just combined.

Stir in hot coffee until the batter is smooth. Divide evenly among the prepared pans.

Bake for 25–30 minutes or until a toothpick inserted in the center comes out clean. Let cool completely.

Meanwhile, mix graham cracker crumbs with sugar and melted butter; set aside.

In a chilled bowl, whip heavy cream until soft peaks form; gently fold in marshmallow fluff and softened butter until smooth.

To assemble, place one cake layer on a serving plate, spread some marshmallow frosting, and sprinkle a layer of graham cracker crunch. Repeat with remaining layers.

Frost the outside of the cake with the marshmallow mixture and press graham cracker crumbs around the sides.

Toast additional marshmallows on top using a kitchen torch or broiler, then decorate with chocolate drizzle.