Preheat your oven to 350 degrees Fahrenheit and prepare three 8-inch round cake pans by lining them with parchment paper and spraying with baking spray.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt.

In a large bowl, whisk the granulated sugar and eggs until combined. Add the buttermilk, oil, and vanilla, whisking until smooth.

Gradually add the dry mixture to the wet ingredients, stirring gently until combined. Finally, whisk in the hot water.

Divide the batter among the prepared pans and bake for 13 to 16 minutes, checking for doneness with a toothpick. Let the layers cool in the pans for 10 minutes before transferring them to a cooling rack.

In the meantime, make the graham cracker crust by combining crushed graham crackers, granulated sugar, brown sugar, and melted butter. Press this mixture into the bottoms of lined pans and freeze for 20 minutes.

Beat the softened butter until fluffy, then add powdered sugar gradually. Mix in marshmallow fluff and vanilla extract until smooth.

Start layering with the graham cracker crust, followed by a layer of chocolate cake, a layer of marshmallow buttercream, and a sprinkle of crushed graham crackers. Repeat until you reach the last layer.

Apply a thin crumb coat of buttercream around the cake and chill for 20 minutes.

Finally, frost the outside with the remaining buttercream and decorate with additional toppings.