In the bowl of a stand mixer with the paddle attachment, combine the flour, sugar, cocoa powder, baking soda, and salt. Add in bits of softened butter, mixing until no longer visible in the dry ingredients and the mixture looks crumbly.

Whisk together eggs, buttermilk, and vanilla. Add to dry ingredients and mix until no dry ingredients are visible. Scrape down the bowl and beat for another 20 seconds.

Fill the three pans evenly - I find a kitchen scale helpful for this part, and each of mine weighed out to about 550g.

Bake for 30-35 min or until a toothpick inserted comes out clean. Allow cake layers to cool for 10-15 minutes on a wire cooling rack before removing from pans, and cool completely before frosting. Set in the fridge or freezer to accelerate the cooling process if desired.

Beat together softened cream cheese and butter; slowly add in powdered sugar alternating with milk until frosting reaches desired consistency. Add vanilla and salt if needed and beat until well combined.

Once your cakes are cool, level them (if needed/desired, these layers bake fairly flat). Place a smear of frosting on your cake circle (to keep the cake from sliding while you decorate it) and center your first cake layer in the center of the circle. Pipe a ring around the outer rim of the cake and fill with marshmallow cream (or spread the layer with frosting). Sprinkle chopped graham crackers and chocolate bars over the filling. Center the next cake layer on top. Repeat the process with your remaining cake layer.

Now you're ready to crumb-coat . If you're unfamiliar with crumb-coating, it's just what it sounds like – spreading a thin layer of frosting over the entire outside of the cake to keep crumbs out of your final layer. I usually do this with my large offset spatula.

Once your crumb coat has set (this takes about 5-10 minutes in the fridge), add your final layer of frosting and smooth. I like to use an offset spatula and bench scraper for this part. Once your cake is covered, place it into the fridge or freezer to set the frosting.

Place chocolate chips in a heat-proof bowl. Add whipping cream and microwave on half power in 30-second intervals, stirring in between, until fully melted and smooth.

With a piping bag or squeeze bottle - or spoon if you'd like - slowly drizzle the ganache around the upper edge of the cake, pausing every inch or so to let more ganache fall in a drip down the side of the cake. Return cake to the fridge to set the drips.

Now to finish decorating - you can use pictures for inspiration or do whatever you want! I decorated with longer/taller crackers and pieces of chocolate bar at the back, and used shorter ones working my way forward. I found toasting marshmallows in the oven on 'low broil' worked the best to toast them without burning them - I tried a brûlée torch but I had trouble browning them without turning them black or setting them on fire. :/

Add more chopped chocolate/graham cracker pieces and large or mini marshmallows at the base of the cake (or leave them off for a more simple look).

Sprinkle on some more chopped chocolate/graham cracker crumbs for good measure - and step back and admire your cake!