Piping bag fitted with a large French star decorating tip; chef's torch
For the cakes: Preheat the oven to 350 degrees F. Coat five 7-inch cake pans with baking spray.
Combine the butter and sugar in the bowl of an electric mixer and beat until light and fluffy. Add the eggs, one at a time, beating well after each addition. Add the baking soda, baking powder and salt; mix well.
In a large glass measuring cup with a spout, combine the hot coffee, buttermilk and dark cocoa powder and whisk until no lumps remain. Add 1 cup of flour at a time to the creamed butter mixture, alternating with additions of the coffee mixture, beginning and ending with flour. Scrape down the sides of the bowl and mix just until combined.
Divide the batter between the prepared cake pans. Bake until a toothpick inserted near the center of the cakes comes out clean, 30 to 35 minutes. Cool the layers in the pans for 5 minutes, then turn the cakes out onto wire racks to cool completely.
For the marshmallow filling: Combine the cream cheese and marshmallow cream in the bowl of an electric mixer. Mix on high speed until well combined. Transfer to a bowl and chill until firm but spreadable, about 2 hours.
For the ganache frosting: Combine the chocolate and heavy cream in a microwave-safe bowl. Microwave on high power until the mixture is steaming but not boiling, about 1 minute. Allow the mixture to stand for 1 minute, then whisk together until smooth and shiny. Cover the bowl with plastic wrap and chill until thick and spreadable, about 1 hour.
For the meringue frosting: Combine the cold water, half of the sugar and the meringue powder in the bowl of an electric mixer. Beat for 5 minutes. Gradually add the remaining sugar and beat until thick and fluffy. Transfer the frosting to a piping bag fitted with a large French star decorating tip.
For assembly: Place a cake layer on a cake server and top with an even layer of marshmallow frosting. Continue to stack and frost until all of the cake layers are used; leave the top layer bare. Chill the cake until the filling is set, about 30 minutes. Cover the chilled cake with a thin layer of chocolate ganache; reserve leftover ganache for later use. Pat graham crackers over the bottom half of the cake with your hands. Break the chocolate bars into pieces and use leftover ganache to attach pieces of the chocolate bars around the bottom edge of the cake, spacing and staggering them about 1/2 inch to 1 inch apart.
Place the marshmallows in a medium microwave-safe bowl. Microwave until hot and well puffed, 30 seconds to 1 minute. Stir briefly and then let sit until cool enough to handle, about 1 minute. Scoop up some of the marshmallow with your fingers (use gloves for easy cleanup) and stretch the marshmallow around the edges and over the top of the cake. Repeat as many times as desired. If the marshmallow begins to set, reheat it for 30 seconds.
Pipe large stars of meringue frosting around the top edge of the cake; also pipe one big swirl on the top center of the cake. Use a chef's torch to toast the meringue. Insert one corner of each of the graham crackers into the middle swirl of toasted frosting so they stand upright, staggering them slightly.
Serve the cake immediately or refrigerate it until ready to serve.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)