Special equipment: two 9-inch round cake pans
For the cake: Position an oven rack in the center of the oven, and preheat to 350 degrees F. Line the bottom of two 9-inch round cake pans with parchment, and grease the parchment and the sides of the pans with oil.
Stir together the cocoa powder and boiling water, and let sit to bloom for 5 minutes (this step intensifies the chocolate flavor in the cake).
Whisk together the granulated sugar, flour, baking powder and salt in a large bowl. Whisk together the bloomed cocoa, oil, milk, eggs and vanilla in a medium bowl. Pour the cocoa mixture into the sugar mixture, and stir until smooth (the batter will be thin). Divide the batter evenly between the prepared cake pans. Bake until the cakes bounce back when pressed in the middle and a toothpick inserted in the center comes out clean, 30 to 40 minutes. Let cool completely in the pans on a rack.
For the filling and frosting: Heat the cream in a double boiler over low heat, whisking occasionally, until it begins to steam. Whisk in the cocoa powder, chocolate, espresso powder, vanilla and salt until the mixture is smooth, about 2 minutes. Whisk in the butter until melted. Whisk in the confectioners' sugar until incorporated. Let the frosting cool completely.
To assemble: Put 1 cake, bottom-side up, on a serving plate or cake stand. Spread about 1 cup of the frosting over the top but not all the way down the side. Top with the other cake, bottom-side up, and frost the top and sides with the remaining frosting.
For the spider web or mummy decoration: Microwave the marshmallows in a microwave-safe medium bowl until they start to grow in size and are soft enough to stir, about 1 minute. Let sit a few minutes until cool enough to touch. Spray your hands with cooking spray. For a spider web, pick up a tablespoon-sized blob of the melted marshmallow and stretch it over and around the cake; repeat so that the strings of marshmallow crisscross one another in many directions, and continue until you have what looks like a spider web. For a mummy, stretch the marshmallow so that all the strings on the top of the cake run in the same direction, leaving a small gap between strings for the mummy's eyes to peek out. Garnish with a candy spider or candy eyes.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
From Food Network Kitchen