Hot Chocolate Layer Cake with Homemade Marshmallows

For the cake

• 6 oz. (3/4 cup) unsalted butter; more for the pans
• 13-1/2 oz. (3 cups) unbleached all-purpose flour; more for the pans
• 3/4 cup canola oil
• 4-1/2 oz. bittersweet chocolate, finely chopped
• 3 cups granulated sugar
• 2-1/4 oz. (3/4 cup) natural unsweetened cocoa powder
• 3 large eggs, at room temperature
• 3/4 cup buttermilk, at room temperature
• 2 Tbs. pure vanilla extract
• 2-1/2 tsp. baking soda
• 1/2 tsp. kosher salt

For the frosting

• 2-1/2 cups heavy cream
• 3 oz. (6 Tbs.) unsalted butter
• 1 vanilla bean, split lengthwise and seeds scraped out
• 6 oz. bittersweet chocolate, finely chopped
• 2 cups granulated sugar
• 6 oz. (2 cups) natural unsweetened cocoa powder; more for decorating
• 1/2 cup Lyle’s Golden Syrup
• 1/4 tsp. kosher salt

For the marshmallows

• Three 1/4-oz. envelopes unflavored powdered gelatin
• 2 cups granulated sugar
• 1 cup light corn syrup
• 1/4 tsp. kosher salt
• 1 tsp. pure vanilla extract
• 1 cup plus 2 Tbs. confectioners’ sugar; more as needed

Make the cake
Position racks in the bottom and top thirds of the oven and heat the oven to 350°F. Butter three 9x2-inch round cake pans and line each with a parchment round. Butter the parchment, then dust with flour and knock out the excess.

In a 3-quart saucepan, combine the butter, oil, chopped chocolate, and 1 cup water. Heat over medium heat until melted.

In a large bowl, whisk the flour, sugar, and cocoa powder. Pour the hot chocolate mixture into the sugar mixture and whisk until combined.
Whisk in the eggs, one at a time, then whisk in the buttermilk, vanilla, baking soda, and salt. Divide the batter evenly among the prepared pans.

Set two pans on the top rack and the third on the lower rack. Stagger the pans on the oven racks so that no pan is directly over another. Bake, swapping and rotating the pans’ positions after 20 minutes, until a toothpick inserted in the center of each cake comes out clean, 35 to 40 minutes. Cool on racks for 10 minutes. Invert the cakes onto the racks, remove the parchment, and cool completely.

Make the frosting
In a 4-quart saucepan over low heat, combine the cream, butter, and vanilla bean and seeds and stir until the butter is melted. Remove the vanilla bean and whisk in the chopped chocolate until melted. Whisk in the sugar, cocoa powder, syrup, and salt until smooth—be sure the cocoa powder dissolves completely. Pour into a 9x13-inch pan and freeze until firm, about 2 hours, or refrigerate overnight.

Make the marshmallows
Pour 3/4 cup cold water into the bowl of a stand mixer. Sprinkle the gelatin over the water. Attach the bowl to the mixer and fit it with the whisk attachment.

Clip a candy thermometer to a 3-quart saucepan; don’t let the tip of the thermometer touch the bottom of the pan. In the saucepan, boil the sugar, corn syrup, salt, and 3/4 cup water over medium heat without stirring until it reaches 234°F to 235°F, about 10 minutes. With the mixer on low speed, pour the hot sugar mixture into the gelatin in a slow, thin stream.

Add the vanilla, carefully increase the speed to high, and beat until the mixture has thickened and cooled, about 5 minutes (the bottom of the bowl should be just warm to the touch). Line a 9x13-inch pan with foil, leaving an overhang on 2 sides. Sift 1 Tbs. of the confectioners’ sugar into the bottom of the pan, then pour the marshmallow mixture into the pan and sift another 1 Tbs. confectioners’ sugar on top. Let sit at room temperature until set, at least 2 hours.

Assemble the cake
Remove the frosting from the freezer or refrigerator. Transfer to the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed for 2 minutes to soften. Change to a whisk attachment and beat at medium-high speed until light and fluffy, about 3 minutes.

Put a cake layer on a flat serving platter or a cake stand lined with strips of waxed paper to keep it clean while icing. Top the layer with 1-1/2 cups of the frosting, spreading it evenly with an offset spatula to the cake’s edge. Repeat with another cake layer and 1-1/2 cups frosting. Top with the last cake layer.

Put 1-1/2 cups of the frosting in a small bowl. With an offset spatula, spread this frosting in a thin layer over the top and sides of the cake. Refrigerate the cake until the frosting firms enough to seal in the crumbs, 20 to 30 minutes.

Spread the remaining frosting in a smooth layer over the top and sides of the cake. If necessary, you can rewhip the remaining frosting to loosen and lighten it. Remove the waxed paper strips.

Use the foil overhang to lift the marshmallow from the pan. Using a knife that has been dipped in cold water, cut along the edge of the marshmallow to release it from the foil. Transfer to a cutting board and remove the foil. Put the remaining 1 cup confectioners’ sugar in a medium bowl. Cut the marshmallow into cubes of different sizes, from 1/4 to 3/4 inch (you will need to continue to dip the knife in cold water as you cut the marshmallows). The marshmallows will be very sticky—dip the cut edges in the confectioners’ sugar to make them easier to handle. As you work, toss a few cubes at a time in the sugar to coat, then shake in a strainer to remove the excess. Mound the marshmallows on top of the cake (you’ll need only a third to half of them). Sift some cocoa powder over the marshmallows.

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