Preheat oven to 350°F. Prepare the three 8-inch cake pans by greasing and flouring the pans. A round of parchment paper may also be used to line the bottom of the pan if desired.
Prepare the graham crumble by adding the ground graham crackers, brown sugar, and salt in a bowl. Mix with a fork to break up any lumps of sugar. Melt butter and pour over the crumb mixture. Combine ingredients to make a crumble. Transfer crumbs out on a parchment lined sheet pan and bake at 350°F for 10 minutes. Remove pan from oven and set aside on a cooling rack to cool completely. When the crumble is cooled, transfer into a bowl, and set aside.
To prepare the graham cakes, in a large bowl add the all-purpose flour, ground graham crackers, baking powder, baking soda, cinnamon, and salt. Use a whisk to incorporate all the ingredients. Set bowl aside. In a bowl with a paddle attachment, add the softened butter and sugars. Cream until light and fluffy. Add the eggs one at a time and vanilla extract. Scrape the sides of the mixing bowl with a rubber spatula after the addition of each egg. Mix on medium speed to ensure the eggs are incorporated in the butter mixture. In three additions, add the dry ingredients to the butter mixture, and then add a portion of the buttermilk. After the remaining ingredients have been added, scrape the sides of the bowl, and mix on low speed to incorporate the dry ingredients. Be careful not to overmix. Divide the batter evenly between two of the three pans and bake for approximately 30-32 minutes, or until a toothpick inserted comes out clean. Remove the cakes from the oven and place on a cooling rack to cool completely before removing from the pan.
To prepare the chocolate cake, in a large bowl, add the all-purpose flour, cocoa, baking powder, and baking soda. Use a whisk and mix to incorporate ingredients. Add sugars to the dry ingredients and stir with whisk to break up any lumps. In another large bowl, combine eggs, oil, sour cream, and vanilla. Using a whisk, blend well. Pour the dry ingredients over the wet ingredients and either mixing by hand with a whisk or with an electric hand mixer, mix on low until just blended. Add the hot coffee to the chocolate mixture in two stages to help to minimize the clumps. Mix until combined and smooth. Pour batter into prepared pan. Bake at 350°F for 32-35 minutes, or until toothpick inserted comes out clean. Remove cake from the oven and place on a cooling rack to cool completely before removing from the pan.
To prepare the buttercream, place all the softened butter in a mixing bowl with a paddle attachment. Cream butter until smooth. In three stages, add the sifted powdered sugar on low speed. After the first addition, add the salt. After each addition of powdered sugar, scrape the sides of the mixing bowl with a rubber spatula. After the last addition of powdered sugar, beat on medium-high to lighten the buttercream. On low speed, slowly pour in the vanilla extract and heavy whipping cream. Once the remaining liquid ingredients have been added, beat on medium speed to lighten the buttercream slightly. Add the marshmallow fluff and beat on low speed to incorporate. Do not overbeat as this will deflate the buttercream. Cover buttercream and set aside until the cake is ready to be assembled.
Prepare the chocolate topping. Place chopped dark chocolate in a microwave-safe bowl. In a glass measuring cup, heat heavy whipping cream in microwave until hot. Be careful not to overheat as the cream will overflow. Pour the hot cream over the chocolate and let sit for a few minutes to melt chocolate. Stir with a rubber spatula and mix until smooth. Set bowl aside.
To assemble cake, remove cakes from the pans and set aside. If needed, cut the tops of the cakes with serrated knife to ensure that the tops are flat. Place one of the graham cake layers on a serving plate or cake stand. Top cake layer with approximately 1 ½ cups of the buttercream. Using an offset spatula, spread buttercream evenly over layer and to the edges. Sprinkle ¼ cup of the graham crumble over the layer of buttercream. Place the chocolate cake layer, with the cut side up, on top of the graham cake layer. Top with another 1 ½ cups of the buttercream and spread buttercream evenly with an offset spatula. Sprinkle another ¼ cup of the graham crumble on top of the buttercream. Place the second graham cake layer with the bottom side up on top of the chocolate cake layer. Using a large offset spatula, ice the cake with the remaining buttercream starting with the top of the cake and then ice the sides. Smooth the sides and top with a bench scraper. Any remaining buttercream may be used for decorations.
To decorate finished cake, press the remaining graham crumble along the bottom of the cake. Pour the chocolate topping in the center of the cake and using an offset spatula, spread topping to the sides, letting the topping drip down. Before topping the cake, if the chocolate topping has cooled too much, warm topping in the microwave. Decorate the top of the cake with the marshmallows to mark where the servings will be. Place twelve marshmallows around the cake to mark the servings, and the four remaining marshmallows may be placed in the center. If desired, a small butane torch may be used to toast the marshmallows. Garnish the top of the cake with the broken-up graham crackers, chopped milk chocolate, and graham crumble.