• Preheat oven to 350°F. Line three 8-inch baking pans with parchment paper circles and set aside.

Graham Cracker Crust

• Mix graham cracker crumbs, butter, and sugar together in a bowl, then divide evenly between the bottom of each pan. Press down, then bake for 8-10 minutes.
  2 1/2 cups graham cracker crumbs, 1/3 cup granulated sugar, 1/2 cup salted butter

Chocolate Cake

• Beat the oil and sugar in the bowl of a stand mixer on medium speed until creamy and light, about 2 minutes.
  1 cup vegetable or canola oil, 2 cups granulated sugar
• Add the eggs and vanilla, mixing well until combined.
  3 large eggs, 2 teaspoons vanilla extract
• In a separate bowl, whisk the flour, cocoa powder, baking soda, baking powder and salt. Add the dry ingredients and buttermilk alternately, half at a time, mixing just until combined each time and scraping the sides of the bowl.
  2 cups all-purpose flour, 1 cup cocoa powder, 2 teaspoons baking soda, 1 teaspoon baking powder, 1 teaspoon salt, 1 cup buttermilk
• Add the hot water right at the end and mix well. Divide the batter evenly between the pans, pouring it right over the graham cracker crumbs.
  1 cup hot water
• Bake for 25-30 minutes at 350°F until a toothpick inserted into the center of the cakes comes out clean with just a few crumbs clinging to it.
• Cool the cake layers completely and level with a sharp serrated knife.

Chocolate Ganache

• Heat the cream in the microwave for 30-60 seconds until hot. Pour over the chocolate chips in a small bowl, then let sit for 5 minutes. Stir until melted and smooth. Let cool while preparing the marshmallow filling.
  1 cup milk chocolate or semisweet chocolate chips, 1/2 cup heavy cream

Toasted Marshmallow Filling

• Set oven to broil with a rack set 2/3 of the way up. Spread the marshmallows in an even layer on a baking sheet lined with parchment paper.
  24 large marshmallows
• Toast the marshmallows until they start to brown (watch them closely! This can happen in just a few seconds!), then remove them from the oven and flip. Toast again on the other side. Let them cool for a few minutes.
• Meanwhile, beat butter, powdered sugar, and vanilla in a large mixing bowl until creamy and smooth.
  1 cup salted butter, 1 cup powdered sugar, 1/2 teaspoon vanilla extract
• Add the cooled marshmallows and jar of marshmallow fluff, then beat well until combined.
  1 jar marshmallow fluff

Chocolate Buttercream Frosting

• In a large bowl, beat the butter and cocoa powder until smooth. Gradually add the powdered sugar, mixing on low speed to combine.
  2 cups salted butter, 1 1/3 cups unsweetened cocoa powder, 5 cups powdered sugar
• Add the vanilla, heavy cream, and pinch of salt and beat well for 3-4 minutes until creamy and smooth. The frosting should be a nice, thick consistency that is spreadable but also holds its shape. Add a little more cream, a teaspoon or two at a time, to get it to the desired consistency.
  6 Tablespoons heavy cream, Pinch of salt, 2 teaspoons vanilla extract
• Use a sturdy spoon or spatula to stir the frosting, pressing it against the sides of the bowl for a few minutes, to knock out some of the air bubbles for a smoother appearance on the sides of the cake.

Marshmallow Fluff

• Whisk egg whites and salt on medium speed in a mixer until frothy. Add 1/3 cup of the sugar, 1 tablespoon at a time, whisking for 20 seconds between each addition.
  3 egg whites, 1/4 teaspoon salt, 1 cup granulated sugar
• Add 2/3 cup of the sugar to a small saucepan along with the corn syrup. Heat over medium-high heat until it reaches 240°F.
  1/2 cup light corn syrup
• With mixer running on medium-low speed, very slow pour the hot sugar mixture in a thin stream over 1-2 minutes until it has all been added. Add the vanilla extract.
  1 teaspoon vanilla extract
• Whisk on medium-high speed for 5-10 minutes until thickened and cooled.

Assembly

• Set bottom cake layer, graham cracker crust side down, on a cardboard or acrylic cake disc for easier decorating, or directly onto a cake plate.
• Transfer some of the chocolate buttercream to a piping bag fitted with a wide tip and pipe a thick ring of frosting around the rim of the cake.
• Spread half of the chocolate ganache on top of the cake inside the ring of frosting. Transfer to the freezer for 5 minutes to set the ganache layer.
• Spread half of the toasted marshmallow filling on top of the ganache layer. Gently place the next layer of cake on top of the marshmallow filling, then repeat piping a ring of chocolate buttercream, filling with the remaining ganche, chilling to set, then adding the remaining toasted marshmallow filling.
• Place the final cake layer on top and frost the outside of the cake with a thin crumb coat of chocolate buttercream frosting. Transfer the whole cake to the freezer for 15-30 minutes to set up.
• Remove the cake from the freezer and top with another acrylic cake disc or cardboard cake round. Frost the outside edges of the cake with the chocolate frosting, using a cake scraper to scrape the sides smooth. You may need to fill in holes with additional frosting and repeat the process of dragging the cake scraper around the cake to get a smooth finish. Place the cake back in the freezer for another 10-15 minutes.
• Use a thin, sharp knife to separate the cake disc or board from the top of the cake. Spread the remaining chocolate frosting on top.
• Assemble graham cracker halves, squares of Hershey's chocolate, and large marshmallows into s'mores and place them on top of the cake, if desired.
  Additional graham crackers, Additional Hershey bars, Additional large marshmallows
• Fill a large piping bag fitted with a large tip with wide teeth with the marshmallow fluff. Pipe the marshmallow fluff all over the top of the cake.
• Use a kitchen torch to toast the marshmallow fluff and marshmallows in the s'mores decorating the top of the cake, then slice and serve.