Begin by preheating the oven to 350 degrees Fahrenheit.

Line four seven-inch or three 8-inch pans with parchment rounds and spray with non-stick baking spray.

Add 2 1/2 cups sugar and 1 cup butter into the bowl of a stand mixer. Cream together on a high speed for 1-2 minutes with a whisk attachment or hand mixer. The mixture should become light in color.

Mix in 4 large eggs on a medium speed, two at a time.

Add 1 tsp baking soda, 1 1/2 baking powder, and 1 tsp salt. Mix on high for another minute.

In a separate bowl, combine 1 cup of hot water, and 1 cup of buttermilk. Pour in 1 cup of black cocoa and whisk until no clumps remain.

Next, add 1 cup of flour into the butter/egg mixture. Scrape the sides of the bowl with a spatula, to ensure the flour is fully incorporated.

Pour in half of the buttermilk/cocoa mixture and gently pulse the mixture to incorporate the liquid (if you don't, you'll end up splattering the cocoa mixture all over you kitchen!)

Mix in the 2nd cup of flour, at a low speed. Pour in the remaining buttermilk/cocoa mixture, and again gently pulse, then mix on low until fully combined.

Add in the last cup of flour, and mix on low until the flour is incorporated.

Evenly divide between the prepared pans. I like to use a kitchen scale to ensure each pan has exactly the same amount of batter.

Bake for 32-35 minutes, or until a toothpick comes out clean.

Allow layers to cool in the pans for about 10 minutes, then run an offset spatula around the edge of the pans to help separate the layers from the pans.

Place the pans in the freezer for about 45 minutes, to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers from the pans.

Use a serrated knife to level the tops of the layers, then set aside. If you make the cake layers in advance, you can wrap and freeze them for up to 3 weeks.

While the cake layers bake and cool, make the chocolate buttercream frosting.

Beat 2 cups of butter on a medium speed for 30 seconds with a paddle attachment or hand mixer, until smooth. 

Mix in 1/2 tsp of salt.

Slowly add in 6 cups of powdered sugar, 1 cup at a time. Half way through adding the powdered sugar, pour in 1/2 cup of heavy cream or milk to make it easier to mix the frosting.

Once all the powdered sugar is incorporated, mix in about 1 cup of marshmallow fluff

Beat on low until the ingredients are fully incorporated, and the desired consistency is reached.

If the frosting is too thick, add an extra Tbsp of heavy cream. If it's too thin, add in another 1/2 cup of powdered sugar.

Stir by hand with a rubber spatula for a couple minutes to get the frosting SUPER smooth.

Place 1 1/2 cups of marshmallow frosting into a large bowl and mix in 1 cup of sifted cocoa powder and 1/2 cup of melted, cooled dark chocolate. Stir until smooth then place into a large piping bag. Seal the top with a rubber band. Snip 1 inch opening at the base of the bag.

Place the rest of the regular marshmallow frosting into a large piping bag and seal the top with a rubber band. Snip 1 inch opening at the base of the bag and set side.

Place 1/2 cup milk chocolate chips in a heatproof bowl and set aside.

Heat 1/3 cup of heavy cream in a heatproof bowl in the microwave for 45 seconds to 1 minute, until gently bubbling.

If you don’t have a microwave or prefer to use the stove top, you can heat the cream over a medium heat in a pan until it begins to simmer. Remove from heat.

Gently pour the heavy cream over the chocolate chips, making sure they are covered with cream. Allow mixture to sit for 1-2 minutes.

Stir slowly until the cream and chocolate are fully combined, and ganache is silky smooth. Pour into a plastic squirt bottle to easily add drips to a cake. If you don’t have any plastic bottles on hand, you can also use a spoon to drizzle it over cakes.

Set aside to cool for about 20 minutes (or place in fridge for about 10 minutes), until the mixture is just slightly warm to the touch.

Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.

Add an even layer of marshmallow buttercream between each cake layer with a large offset spatula. Top with mini toasted marshmallows, graham cracker crumbs, and a drizzle of chocolate ganache.

Repeat with remaining cake layers.

Once the cake layers are stacked, spread a thin coat of frosting around the the cake, to fully cover the cake layers.

Smooth using a bench scraper, then chill the cake in the fridge (20 minutes) or freezer (10 minutes) until the frosting is firm to the touch.

Pipe alternating stripes of chocolate and marshmallow frosting around the cake and smooth using a bench scraper.

Chill the cake in the fridge (20 minutes) or freezer (10 minutes) until the frosting is firm to the touch again before adding the chocolate drip.

Once the cake is chilled, complete a test drip to check the consistency of you ganache. If the test drip seems too thick, try heating up the ganache for 5-10 additional seconds in the microwave. If the mixture seems too thin, allow it cool a bit longer, then try another test drip.

Top the cake with mini s'mores and enjoy!