- 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
- 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.
- 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.
- 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.
- In a small bowl, sprinkle the gelatin over the cold water and let stand until softened, about 5 minutes.
- 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.
- 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.
- 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.
- 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.
The cakes can be prepared through Step 3 and kept refrigerated for up to 4 days. Bring to room temperature before proceeding.