Preheat oven to 350º F (325º F for dark or nonstick pan) and grease pans.
In a large bowl, combine cake mix, eggs, water, and vegetable oil with mixer on medium speed or beat vigorously by hand for two minutes until all the ingredients are fully incorporated.
Distribute cake batter into three 9” round pans as evenly as possible. These cakes will be on the thinner side. Use a spatula or a spoon to smooth batter evenly across the bottom of the cake tin if necessary.
Bake for approximately 15 minutes, using a butter knife to frequently check the cakes’ doneness. Any cakes on the bottom rack closest to the heating element will most likely need to be removed before the top-rack cakes.
Once all three cakes are removed from the oven, leave in tins to cool for 10 minutes. Then, transfer to cooling racks for 20 minutes.
In a large bowl, crush graham crackers into bite-sized crumbs. Add marshmallows and chocolate chips. Toss to combine, and set aside.
Spoon your chocolate frosting into a Ziploc bag. Squeeze frosting to a bottom corner, twisting the rest of the bag tight. Snip off the point of the bag’s corner to create a piping hole.
Place a cooling rack on a sheet pan and set on a sturdy, easy-to-work-on surface. (Cover the sheet pan with parchment paper for easy cleanup.)
Once your cakes are completely cooled, you can begin layering. Because not all three of my cake rounds were perfectly even, I’ll be stacking my rounds biggest to smallest from the bottom up. Place your foundational layer on the cooling rack on the sheet pan.
Use a butter knife to carefully slice off the uppermost layer of your cake, creating a level surface for frosting and filling.
Pipe a thick ring of chocolate frosting around the edge of your cake. This will act as a “border” for your s’mores filling and will help create a sleek silhouette once the ganache is added.
Add an even layer of s’mores mix on the cake layer, staying within the line of the chocolate frosting.
Carefully set the next cake round on the first, and repeat steps #10-12. Add the final cake round to the second layer, but don’t shave anything off the top.
Fill any gaps between your layers with chocolate frosting. Use a butter knife or offset spatula to create a smooth, even surface around the cake.
Pour one cup of semi-sweet chocolate chips into a small bowl. Set aside.
In a small saucepan, bring heavy whipping cream to a low simmer. Keep a close eye on the cream and remove from heat as soon as small bubbles begin to appear around the edges.
Pour heavy whipping cream over chocolate chips. Stir once to make sure all the chips are covered. Cover the bowl with a pot lid and leave undisturbed for five minutes.
After five minutes, whisk your chocolate and cream starting from the middle and working your way out. Continue whisking until ingredients are fully incorporated and chocolate is smooth and velvety. Let sit undisturbed for 15 minutes.
Starting in the center, pour approximately ? of the ganache over the cake, allowing it to slowly spread over and down the cake’s sides. Carefully pour the rest of the ganache over the cake, paying special attention to any bald spots or gaps in the frosting. Let cake sit for 10-15 minutes.
Add leftover s’mores mix to the center of your cake. Decorate with two miniature Hershey’s bars.
For an extra decorative touch, crush the remaining graham crackers of your s’mores mix into a fine powder. Sprinkle powder over the outer edges and sides of your cake for a delicate dusting of sugar and cinnamon.
Using a box mix and pre-made frosting is convenient, but if you’re a traditionalist, feel free to substitute either ingredient for homemade. Here is a great guide to making buttercream frosting from scratch. If you’re feeling particularly experimental, try out different flavors like classic vanilla, peanut butter, Swiss meringue, or cream cheese.
Check out this guide to creating your best chocolate cake yet. (Hint: it can be as easy as swapping out a floured cake tin with a tin dusted in cocoa powder!)
This cake can be made in round or square tins of any size. If you plan on changing the size or number of your layers, keep an eye on your cakes as they bake to prevent over/underbaking. The taller/thicker the cake, the longer the cake will need in the oven. Thin cakes can burn quickly if baked too long. I wouldn’t recommend going any taller than four layers without structural reinforcement.