Prepare the Cake Batter : Preheat your oven to 350°F (175°C). Grease and line three 8-inch round cake pans with parchment paper. In a large mixing bowl, sift together the flour, cocoa powder, baking powder, baking soda, granulated sugar, and salt. In a separate bowl, whisk together the buttermilk, eggs, oil, vanilla extract, and coffee. Slowly add the wet ingredients to the dry ingredients, mixing until smooth and lump-free.

Bake the Cake Layers : Divide the batter evenly among the prepared cake pans. Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

Make the Marshmallow Filling : In a mixing bowl, combine the softened butter, marshmallow creme, powdered sugar, and a pinch of salt. Beat until smooth and fluffy. Set aside half of the marshmallow mixture for spreading between the cake layers and reserve the rest for topping the cake.

Make the Ganache : Heat the heavy cream in a small saucepan over medium heat until it begins to simmer (do not boil). Pour the hot cream over the dark chocolate in a heatproof bowl and let it sit for 2–3 minutes. Whisk until smooth and glossy.

Assemble the Cake : Place the first cake layer on a serving plate or cake stand. Spread a layer of marshmallow cream on top and sprinkle with crushed graham crackers. Repeat this process for the second cake layer. Place the final cake layer on top and spread a thin crumb coat of marshmallow cream over the entire cake. Chill for 20 minutes to set.

Finish with Ganache and Topping : Pour the ganache over the top of the chilled cake, letting it drip down the sides for a dramatic effect. Spread the reserved marshmallow cream on top of the cake, creating peaks with a spatula. For an authentic s'mores touch, use a kitchen torch to lightly toast the marshmallow topping.

Garnish and Serve : Garnish with additional graham cracker crumbs, chocolate shavings, or mini marshmallows, if desired. Slice and serve your decadent S'mores Cake to the delight of your guests!