Let’s build this incredible S’mores Chocolate Cake layer by delicious layer! First, prepare the oven and pans for the cake. Preheat oven to a lower temperature: 300°F (148°C). Grease three 8-inch round cake pans and line the bottoms with parchment paper circles. Set aside.

Make the chocolate cake batter. In a large bowl, whisk together the flour, granulated sugar, sifted cocoa powder, baking soda, salt (and optional baking powder). Ensure everything is well combined. In a separate medium bowl, whisk together the room temperature eggs, room temperature milk, vegetable oil, and vanilla extract. Pour the wet ingredients into the dry ingredients and mix with a whisk or electric mixer on low speed until just combined. Carefully stir in the hot water (or coffee) until the batter is smooth and thin.

Divide the thin batter evenly among the three prepared cake pans. Bake in the preheated 300°F (148°C) oven for 30-33 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. The lower temperature encourages even baking. Let cakes cool in pans for 5 minutes, then invert onto wire racks, remove parchment, and cool completely.

While cakes cool, make the Graham Cracker Crunchies. Increase oven temperature to 350°F (175°C). Line a cookie sheet with parchment paper or a silicone mat. In a small bowl, combine the graham cracker crumbs, 1 1/2 tbsp sugar, and melted butter. Stir until well combined and all crumbs are moistened. Spread the mixture evenly onto the prepared cookie sheet. Bake for 8-10 minutes until lightly golden and crisp. Let cool completely on the baking sheet, then break into small crunchy pieces. Set aside.

Prepare the Toasted Marshmallow Frosting. Spread the 1 1/2 cups of mini marshmallows in a single layer on a baking sheet lined with foil or parchment (lightly greased might help prevent sticking). Place under a preheated broiler (keep oven door ajar and watch constantly) for about 1 minute, or just until the tops are golden brown and toasted. Remove immediately – they burn fast! Let the toasted marshmallows cool completely on the baking sheet; they will deflate and might be sticky.

In a large mixer bowl (stand mixer with paddle or handheld), beat the room temperature butter on medium-high speed until smooth and creamy (2-3 minutes). Add the marshmallow fluff and beat until well combined and smooth. Turn mixer to low and add about half of the sifted powdered sugar. Mix until incorporated, then add the remaining powdered sugar and mix until combined and smooth. Break the cooled, toasted marshmallows into smaller pieces (they might stick together) and add them to the frosting. Mix on low or medium speed just until the toasted marshmallow pieces are incorporated. Don’t overmix or they might completely dissolve. Set frosting aside.

Make the Chocolate Ganache. Place the semi-sweet chocolate chips in a medium heatproof bowl. Heat the heavy whipping cream in a small saucepan or microwave until it just begins to boil or simmer vigorously around the edges. Pour the hot cream immediately over the chocolate chips. Let sit undisturbed for 3-5 minutes. Whisk gently until the ganache is completely smooth and glossy. Set aside to cool and thicken slightly (it should still be pourable for the filling layer but not hot).

Make the Vanilla Frosting Dam. In a medium bowl, beat the room temperature butter until smooth and creamy. Gradually beat in the sifted powdered sugar. Add the vanilla extract and 1 tablespoon of milk or cream. Beat until smooth and fluffy. Add slightly more milk/cream only if needed to reach a pipeable consistency, slightly stiff to hold back the fillings. Transfer to a piping bag with a large round tip or snip a 1/2-inch opening.

Assemble the S’mores Cake! Level the tops of the completely cooled chocolate cake layers if needed. Place the first layer on your serving plate or cake stand. Pipe a dam (a thick ring) of the vanilla frosting around the top outer edge of this layer. Spoon about 6 tablespoons (or about 1/3) of the slightly cooled chocolate ganache inside the dam and spread it evenly. Gently spoon about 1 cup (or about 1/3) of the Toasted Marshmallow Frosting on top of the ganache layer and spread it carefully. Sprinkle about 1/2 cup (or about 1/3) of the Graham Cracker Crunchies over the marshmallow frosting.

Carefully place the second cake layer on top. Repeat the process: pipe a vanilla frosting dam, spread with ganache, spread with marshmallow frosting, sprinkle with crunchies. Place the third and final cake layer on top. Place the stacked cake in the refrigerator for about 30 minutes to help the layers stabilize.

Remove the chilled cake. Apply a thin crumb coat of the Toasted Marshmallow Frosting all over the outside of the cake if desired (optional, but helps for a cleaner finish). Chill again for 15 minutes. Frost the top and sides of the cake smoothly (or rustically) with the remaining Toasted Marshmallow Frosting.

Take the remaining Graham Cracker Crunchies and gently press them onto the bottom 1-2 inches of the frosted cake sides all the way around.

Prepare the Chocolate Drip. Gently reheat the remaining chocolate ganache in the microwave (10-15 second bursts at 50% power) or over warm water, stirring until it reaches a cool but fluid drip consistency. Using a spoon or squeeze bottle, apply the ganache drip around the top edge of the chilled cake. Pour any remaining ganache in the center top if desired and spread carefully.

Just before serving, pile some fresh mini marshmallows decoratively on top of the ganache. Using a kitchen torch, carefully toast the marshmallows until golden brown and slightly gooey. Be careful not to burn them or melt the ganache/frosting underneath too much. Serve relatively soon after torching for the best effect.