S'mores Cake

Instructions

• Gather all ingredients.
• Preheat the oven to 350 degrees F (175 degrees C). Line the bottom of 2 (9-inch) cake pans with parchment paper, lightly spray with baking spray.
• Combine cake mix, eggs, milk, and butter in a large bowl. Beat with an electric mixer until well combined. Mix in espresso powder and 2 teaspoons vanilla extract. Divide batter evenly between prepared pans, spreading into even layers.
• Bake in the preheated oven until a wooden pick inserted into centers of cake comes out clean, 25 to 30 minutes. Cool cakes in pans on a wire rack for 20 minutes, then remove cakes from pans. Let cool completely on a wire rack, about 1 hour.
• While cakes are baking, place chocolate chips into a heatproof bowl. Bring cream in a small saucepan to a gentle simmer over medium-low heat, stirring constantly, about 6 minutes. Pour hot cream over chocolate chips and let stand for 5 minutes. Gently stir together in a circular motion to evenly blend chocolate into cream, without adding a lot of air. Let chocolate mixture cool, uncovered, at room temperature, stirring occasionally, until thickened to spreadable consistency, about 1 1/2 hours.
• Meanwhile, bring a small saucepan filled with 2 inches water to a simmer over medium-low heat. Beat together egg whites, sugar, and cream of tartar in a work bowl of a stand mixer fitted with a whisk attachment on medium speed until well combined, about 1 minute.
• Place mixer bowl over simmering saucepan, ensuring the bottom of the mixing bowl does not touch the water. Cook egg white mixture, whisking constantly, until a candy thermometer reaches 160 degrees F (70 degrees C) and the sugar crystals have dissolved, 8 to 10 minutes. Remove bowl from heat, and return to stand mixer.
• Beat on high speed, gradually adding remaining 1 teaspoon vanilla. Beat until stiff peaks form, 6 to 7 minutes.
• Set aside 2 cups of marshmallow cream. Place remaining marshmallow cream into a piping bag fitted with a large star tip.
• Remove cooled cakes from pans and discard parchment. Using a serrated knife, trim the tops of the cooled cakes to create a level top; use excess cake for trifle or just eat.
• Place 1 cake layer on a cake stand or plate, cut side up. Spread reserved 2 cups marshmallow cream in an even layer on cake top.
• Sprinkle 1/2 cup of the graham cracker crumbs evenly over top.
• Top with remaining cake layer, cut-side down. Fill in any empty spots in seams of cake layers using marshmallow cream in piping bag; smooth sides using an offset spatula. Refrigerate stacked cake, uncovered, for 30 minutes.
• Spoon 1 1/4 cups cooled chocolate mixture over top of the stacked cake. Spread into an even layer using a large cake spatula, allowing excess to extend over the sides. Frost sides of cake using remaining chocolate mixture, smoothing into an even layer.
• Using marshmallow cream in piping bag, pipe 10 large rosettes along the top, outside edge of the cake.
• Sprinkle center of the cake with remaining 1/4 cup graham cracker crumbs.
• Arrange graham cracker rectangles upright on the long sides of each rectangle in a spoke-wheel design inside rosette rings on top of cake. Pipe 1 large rosette in center of graham cracker spoke-wheel design. Refrigerate, uncovered, until ready to serve, at least 30 minutes or up to 48 hours.
• Just before serving, use a kitchen torch to slightly brown marshmallow rosettes. Slice and serve cake.