2 cups all-purpose flour, plus more for dusting
2 cups sugar
2/3 cup unsweetened cocoa powder
1 teaspoon baking powder
1 teaspoon baking soda
1 teaspoon kosher salt
1 1/2 cups milk
1/2 cup vegetable oil
2 extra-large eggs
2 tablespoons pure vanilla extract
1 1/2 teaspoons unflavored gelatin
2 tablespoons cold water
1 cup sugar
3/4 cup light corn syrup
3 large egg whites
1/2 vanilla bean, split and seeds scraped
6 whole graham crackers, broken
2 ounces bittersweet chocolate, melted
1. Preheat the oven to 325°. Butter two 8-inch square baking pans and line the bottoms with parchment paper. Butter the paper and dust the pans with flour, tapping out the excess.
2. In a large bowl, whisk the 2 cups of flour with the sugar, cocoa powder, baking powder, baking soda and salt. In a medium bowl, whisk the milk with the vegetable oil, eggs and vanilla. Add the liquid to the dry ingredients and whisk just until smooth.
3. Scrape the batter into the prepared pans. Bake for about 40 minutes, until a toothpick inserted in the center of the cakes comes out with a few crumbs attached. Let cool in the pans on a wire rack, then turn out to cool completely. Peel off the parchment. Turn the cakes right side up onto 2 baking sheets.
4. In a small bowl, sprinkle the gelatin over the cold water and let stand until softened, about 5 minutes.
5. In a medium saucepan, combine 1/2 cup of the sugar with the corn syrup and stir over moderate heat until the sugar is thoroughly dissolved. Bring the corn syrup mixture to a boil, wiping down any sugar crystals that may form on the side of the pan with a moistened pastry brush. Cook over moderate heat, without stirring, until the corn syrup mixture reaches 250° on a candy thermometer, about 10 minutes. Remove from the heat and stir in the softened gelatin; the mixture will foam up.
6. Meanwhile, in the bowl of a standing electric mixer fitted with a whisk, beat the egg whites and vanilla seeds at medium speed until soft peaks form. Add the remaining 1/2 cup of sugar, 1 tablespoon at a time, beating well between additions. Beat until the whites are stiff and glossy, about 5 minutes.
7. With the mixer at medium speed, carefully pour the hot corn syrup mixture in a thin stream into the egg whites, aiming for the area between the whisk and side of the bowl. Beat the topping until very thick and opaque, about 8 minutes.
8. Preheat the broiler and position a rack 8 inches from the heat. Using an offset spatula, spread the marshmallow topping over the tops of the cakes. Let stand for about 1 minute, until slightly set. Broil the cakes one at a time, shifting the pan as necessary for even browning, about 5 minutes. Insert the broken graham crackers into the marshmallow and drizzle with the melted chocolate. Serve while the marshmallow is still warm.