Make the ganache: Heat the heavy cream, chocolate and salt in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water), stirring occasionally, until the chocolate melts and the ganache is smooth. Stir in the vanilla. Remove the bowl from the pan and set aside until the ganache is cool and thick but still pourable, about 1 hour.
Meanwhile, make the filling: Preheat the oven to 350 degrees F. Toss the graham cracker crumbs, melted butter and salt in a bowl until combined. Spread out on a baking sheet. Bake, stirring occasionally, until toasted, 8 to 10 minutes; let cool. Assemble the cake as close to serving time as possible: Using a long serrated knife, carefully slice each cake layer in half horizontally. Place 1 cake half on a platter. Using a lightly oiled offset spatula or spoon, spread one-third of the marshmallow cream on top, stopping about 1 inch from the edge (if the marshmallow cream is difficult to spread, microwave 10 to 15 seconds to soften). Sprinkle one-third of the graham cracker mixture over the marshmallow cream. Repeat to make 4 layers, ending with cake; reserve a few tablespoons of the graham cracker mixture for topping. Pour the ganache over the cake, letting it drip down the sides. Sprinkle with the reserved graham cracker mixture. Serve immediately.
Basic Chocolate Cake
Preheat the oven to 350 degrees F. Coat two 9-inch-round cake pans with cooking spray and line the bottoms with parchment paper.
Whisk the cocoa powder and 1 1/2 cups boiling water in a medium bowl until smooth; set aside. Whisk the flour, sugar, baking powder, baking soda and salt in a large bowl until combined. Add the eggs, vegetable oil, sour cream and vanilla and beat with a mixer on medium speed until smooth, about 1 minute. Reduce the mixer speed to low; beat in the cocoa mixture in a steady stream until just combined, then finish mixing with a rubber spatula. (The batter will be thin.)
Divide the batter between the prepared pans and tap the pans against the counter to help the batter settle. Bake until a toothpick inserted into the middle comes out clean, 30 to 40 minutes. Transfer to racks and let cool 10 minutes, then run a knife around the edge of the pans and turn the cakes out onto the racks to cool completely. Remove the parchment. Trim the tops of the cakes with a long serrated knife to make them level, if desired.