Let’s break down this multi-component cake process:

1. Make the Chocolate Cake Layers:

• Preheat oven to 300°F (148°C). Prepare three 8-inch cake pans with parchment circles and grease the sides.
• Add all dry cake ingredients (flour, sugar, cocoa, baking soda, salt) to a large bowl and whisk together.
• Add eggs, milk, and vegetable oil to the dry ingredients and mix well.
• Add vanilla to the hot water, then add this mixture to the batter. Mix well. The batter will be thin.
• Divide batter evenly between the prepared cake pans. Bake for 30-33 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
• Cool cakes in pans for about 5 minutes, then invert onto cooling racks to cool completely.

2. Make the Graham Cracker Crunchies:

• Once cakes are cool, preheat oven to 350°F (175°C). Line a cookie sheet with parchment or a silicone mat.
• Combine graham cracker crumbs, sugar, and melted butter. Stir well.
• Spread crumbs evenly onto the prepared cookie sheet.
• Bake for 8-10 minutes, then allow to cool completely. Set aside.

3. Make the Toasted Marshmallow Frosting:

• Arrange the 1 1/2 cups mini marshmallows on a baking sheet and toast under the broiler for about 1 minute, watching very carefully until toasted. Remove and let cool completely.
• In a large mixer bowl, beat the room-temperature butter until smooth.
• Add the marshmallow fluff and beat until well combined.
• Add about half of the powdered sugar and mix until smooth.
• Add the remaining powdered sugar and mix until smooth.
• Break the cooled, toasted marshmallows into smaller pieces (if sticky) and add them to the frosting. Mix until well combined. Set aside.

4. Make the Chocolate Ganache:

• Place the chocolate chips in a medium-sized heatproof bowl.
• Heat the heavy whipping cream in a small saucepan or microwave until it just begins to boil/simmer.
• Pour the hot cream over the chocolate chips. Let sit for 3-5 minutes.
• Whisk gently until smooth and glossy. Set aside to cool slightly.

5. Make the Vanilla Frosting (for Dam):

• In a large mixer bowl, beat the room-temperature butter until smooth.
• Add about half the powdered sugar and mix until smooth.
• Add the vanilla extract and cream/milk and mix until combined.
• Add the remaining powdered sugar and mix until smooth and combined. Transfer to a piping bag (optional, but helpful for creating the dam).

6. Assemble the Cake:

• Level the cooled cake layers using a large serrated knife if needed.
• Place the first cake layer on a cake plate or cardboard circle. Pipe a dam (a ring) of vanilla frosting around the outer edge.
• Spread about 6 tablespoons of chocolate ganache evenly inside the dam.
• Add about 1 cup of marshmallow frosting on top of the ganache and spread evenly.
• Sprinkle 1/2 cup of graham cracker crunchies over the marshmallow frosting.
• Add the second cake layer. Pipe another dam of vanilla frosting. Layer with ganache, marshmallow frosting, and the remaining 1/2 cup of graham cracker crunchies.
• Top with the final layer of chocolate cake.
• Chill the cake for about 30 minutes to stabilize.

7. Frost and Decorate:

• Use the remaining marshmallow frosting to frost the outside of the chilled cake. Aim for a smooth finish using an offset spatula and potentially a bench scraper.
• Press the remaining graham cracker crunchies onto the bottom third or half of the frosted cake sides.
• Gently reheat the remaining chocolate ganache in the microwave if it has firmed up too much. Pour it over the top edge of the cake, using a spoon or squeeze bottle to create drips down the sides.
• Arrange some extra mini marshmallows on top of the ganache. If desired and available, use a kitchen torch to carefully toast the marshmallow garnish.
• Refrigerate the cake until ready to serve.

This assembly requires patience but creates impressive results!