Step 1: Bake the Chocolate Cake
- Preheat oven to 350°F (176°C). Line two 8-inch cake pans with parchment paper and grease the sides.
- In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, and salt. Add egg, buttermilk, oil, and vanilla; mix until smooth. Stir in boiling water carefully. The batter will be thin.
- Divide the batter evenly into pans and bake for 25–30 minutes, or until a toothpick comes out with crumbs. Cool completely on a wire rack.
Step 2: Make the Marshmallow Ice Cream
- Beat cream cheese and sugar until smooth. Add milk and vanilla; mix well. Whip heavy cream with powdered sugar until stiff peaks form. Gently fold whipped cream into the cream cheese mixture in batches.
- Fold in crumbled graham crackers and swirl in marshmallow fluff lightly to create ribbons.
Step 3: Assemble the Cake
- Line an 8-inch springform pan with parchment paper extending above the edges. Place one cake layer at the bottom of the pan.
- Microwave heavy cream until hot; pour over chocolate chips and corn syrup for fudge. Let sit for 3–5 minutes, then whisk smooth. Spread half over the cake layer and top with marshmallows.
- Add half of the ice cream mixture on top; smooth evenly. Repeat layers with remaining cake, fudge, marshmallows, and ice cream. Freeze for 6–8 hours.
Step 4: Frost and Garnish
- Whip heavy cream with powdered sugar, cocoa powder, and vanilla until stiff peaks form for frosting. Remove cake from pan; frost evenly with whipped cream frosting.
- Press crumbled graham crackers onto sides, drizzle chocolate sauce on top, and garnish with marshmallows. Use a kitchen torch to toast marshmallows lightly if desired.