Preheat oven to 300°F (148°C). Line three 8-inch pans with parchment and spray the sides.
Combine dry cake ingredients in a large bowl. Whisk to blend.
Add milk, oil, vanilla, and eggs. Mix until smooth.
Pour in hot water and mix on low until combined. Batter will be thin.
Divide into pans and bake for 30-33 minutes.
Cool for 10 minutes in the pan, then turn out onto racks to cool completely.
Mix graham cracker crumbs, melted butter, and sugar in a bowl.
Press into a parchment-lined 8-inch pan. Bake at 350°F (176°C) for 8-10 minutes. Cool completely.
Heat the cream until hot, pour over chocolate chips, and let sit 2 minutes. Stir until smooth and set aside.
Beat butter until smooth. Add marshmallow fluff and mix well.
Add vanilla and powdered sugar. Beat until fluffy.
Assemble: Cake layer, ganache, frosting, graham cracker layer. Repeat. Frost top with remaining marshmallow frosting.
Toast the top with a kitchen torch if you’re feelin’ fancy. Slice and devour.