Preheat your oven to 350°F (175°C). Line three 8-inch baking pans with parchment paper circles.

In a bowl, combine graham cracker crumbs, melted butter, and granulated sugar. Mix until well combined. Divide this mixture evenly between the bottom of each pan, pressing it down firmly. Bake for about 8-10 minutes.

In a stand mixer, beat vegetable oil and granulated sugar on medium speed until creamy and light, about 2 minutes.

Add eggs and vanilla extract to the mixture, beating well.

In a separate bowl, whisk together all-purpose flour, cocoa powder, baking soda, baking powder, and salt. Incorporate dry ingredients and buttermilk alternately into the mixer.

Just before mixing is done, add in hot water. Mix until smooth.

Divide batter evenly between prepared baking pans, pouring it directly over the graham cracker crusts.

Bake in the preheated oven for 25-30 minutes. Check with a toothpick for doneness.

Allow cake layers to cool completely and level tops for even stacking.

Heat heavy cream in the microwave until hot. Pour over milk chocolate chips, let sit for 5 minutes to melt, then stir until smooth.

Set oven to broil and position a rack about 2/3 way up. Spread marshmallows on a baking sheet lined with parchment and toast until browned.

In a large mixing bowl, beat room temperature butter, powdered sugar, and vanilla extract until creamy.

Add cooled toasted marshmallows and marshmallow fluff to the butter mixture, beat until fully combined.

In a bowl, beat room temperature butter and cocoa powder until smooth.

Gradually add powdered sugar on low speed and then mix in vanilla, heavy cream, and pinch of salt for another 3-4 minutes.

In another bowl, whisk egg whites and salt until frothy. Gradually add 1/3 cup sugar.

In a saucepan, heat remaining 2/3 cup sugar and corn syrup until it reaches 240°F (115°C).

While the mixer is running, slowly pour hot mixture into egg whites to create meringue.

Place the first cake layer on a cake disc. Pipe a thick ring of chocolate buttercream and spread half of the chocolate ganache in the center.

Freeze for 5 minutes to set, then add half of the toasted marshmallow filling.

Repeat with the second cake layer, ganache, and marshmallow filling.

Place the final cake layer on top and apply a thin crumb coat of chocolate buttercream. Freeze for 15-30 minutes.

Frost the outside with remaining chocolate frosting and return to the freezer for 10-15 minutes.

Pipe marshmallow fluff over the top and toast with a kitchen torch. Decorate with graham crackers and Hershey squares if desired.

Slice and serve immediately.