Spray 3-9 inch pans with cooking spray and then line bottom with parchment. Heat oven to 365 degrees.
Combine the dry ingredients in a medium bowl.
Combine milk, oil, eggs and vanilla in another bowl.
Slowly add the wet ingredients into the dry until just combined. Add the hot coffee and stirto combine.
Pour the batter into pans, dividing evenly. Bake for about 30-35 minutes or until a toothpickcomes out of the center clean. Cool in pans about 10 minutes, then transfer the cakes to a cooling rack to cool completely.
To prepare marshmallow
Combine egg whites, sugar and cream of tartar in a heat proof bowl set over a pot of gentlysimmering water (I used the stand mixer bowl to reduce amount of bowls needed).
Heat the mixture, while stirring constantly, until an instant thermometer reads 160 degrees, about 5 minutes.
Remove the bowl from the heat and using the whisk attachment of your stand mixer, mix onhigh speed until stiff, shiny peaks form, about 5 min. Mix in the vanilla.
To prepare graham cracker crumbs
Preheat oven to 350 degrees. Combine all ingredients in a medium bowl, mixing well. Place crumb mixture on a rimmed baking sheet and bake for about 10 minutes until golden brown. Once slightly cooled, crumble into pieces.
To prepare nutella buttercream
Cream the butter, sugar, and nutella together until combined. Add vanilla and whipping cream and beat on high until light and fluffy.
To assemble the cake
Once cooled, level the cakes using a knife. Fill 2 piping bags or zip top bags with the nutella frosting and the marshmallow.
Place the first cake layer on a cake plate. Pipe a thick ring of nutella buttercream around the edge of the cake. Pipe and fill the marshmallow mixture into the center of the cake. Sprinkle about ¾ cup of graham cracker crumbs over the marshmallow and nutella buttercream. Place the second cake layer and repeat the above steps. Place the third cake layer on top. Using the nutella buttercream, coat the outside and top of the cake. Decorate the cake with any remaining graham cracker crumbs and marshmallow. If you have a kitchen torch, toast any marshmallow on the top of the cake.
The easiest way to transfer the frosting and marshmallow into piping bags or zip top bags is to place the bag inside a tall glass with the top of the bag folded over the edge of the glass. Spoon in the frosting/filling and then unfold the top of the bag from the edge of the cup and push the frosting/filling down to the bottom of the bag. If using a zip top bag, snip the corner of the bag last.
To easily frost a cake without getting crumbs in the frosting, place a thin layer of the frosting over the entire cake and then refrigerate the cake for 30 minutes to an hour to chill. Then apply a thicker layer of frosting over top of the thin layer, smoothing as you go.
You could make this in 3-8 inch pans but they will be slightly more full and thicker layers.