Serves: Makes 10 servings
- 7 ounces semisweet or bittersweet chocolate (up to 60 percent cacao), finely chopped
- 1 tablespoon unsalted butter, cut into 4 pieces
- 1 cup (8 ounces) heavy whipping cream
- 1½ cups (7½ ounces) unsalted roasted peanuts
- ¼ cup (1¾ ounces) plus 1¼ cups (8¾ ounces) granulated sugar
- 1 tablespoon (¼ ounce) cornstarch
- 8 large egg whites
- ¼ teaspoon cream of tartar
- 15 tablespoons (7½ ounces) unsalted butter, very soft, but not melted
- 1¼ cups (5 ounces) unsifted confectioners' sugar
- 1 cup minus 2 tablespoons (9 ounces) creamy salted peanut butter, at room temperature
- 1 cup finely chopped store-bought peanut brittle, or finely chopped additional peanuts
- Make the ganache: Place the chocolate and butter in the medium bowl. Bring the cream to a boil in the small saucepan over medium heat. As soon as the cream boils, pour it over the chocolate. Let sit for 1 minute, then whisk until blended and smooth. If you are using a high-percentage chocolate and the mixture looks broken or curdled at this point, stir in an extra tablespoon or two of cream, just until the mixture smooths out again. Cover with plastic wrap and set aside for 2 to 3 hours or up to overnight, until the ganache has cooled completely and has the texture of frosting.
- Make the meringue layers: Preheat the oven to 350°F and position an oven rack in the center. Lightly coat the baking sheet with butter or high-heat canola-oil spray, line it with parchment paper, then lightly coat it with butter or spray again, dust it with flour, and tap off the excess. Place the peanuts, the ¼ cup granulated sugar, and the cornstarch in the bowl of the food processor and process until the nuts are finely ground, 20 to 30 seconds. Place the egg whites and cream of tartar in the bowl of the stand mixer with the whisk and whip on medium-high speed until soft peaks form, 1½ to 2 minutes. You can also use a hand mixer and a medium bowl, although you may need to beat the mixture a little longer to achieve the same results. With the mixer running, slowly rain in the remaining 1¼ cups granulated sugar, then turn the speed to high and whip until the meringue is very stiff, 30 to 45 seconds. Carefully fold the nut mixture into the meringue with the silicone or rubber spatula until no more patches of dry ingredients remain. Spread the meringue evenly in the prepared baking sheet using the offset spatula.
- Bake the meringue for 25 minutes, or until it is golden in color, the center feels firm to the touch, and the edge begins to pull away from the side of the pan. Transfer to a rack and cool completely.
- Make the buttercream: Place the butter in the bowl of the stand mixer with the paddle and beat on medium-low until smooth, about 20 seconds. You can also use a hand mixer and a medium bowl, though you may need to beat the mixture a little longer at each step to achieve the same results. Add the confectioners' sugar and beat, using the cleaned silicone or rubber spatula to scrape down the bowl once halfway through, until smooth and blended, 2 to 3 minutes. Scrape down the bowl again. Add the peanut butter and beat until smooth, another minute or so. Scrape down the bowl and make sure everything is well blended. Set aside ½ cup for piping. The remaining buttercream will be used for filling the layers.
- Cut the meringue: Run a knife around the edges of the baking sheet to loosen the meringue. Lift the meringue out of the pan, using the parchment paper as support, and transfer to a cutting surface. Position the meringue so that one of the long sides is parallel to the edge of your work surface. Use the serrated knife to cut the sheet crosswise through the parchment, into 4 pieces, each 11 by 3¼ inches. Leave each piece attached to the parchment.
- Assemble the marjolaine: Transfer one piece of meringue to the cake cardboard or serving platter and place it top side down. Peel off the parchment paper. With the offset spatula, spread ½ of the buttercream in an even layer over the meringue. Top with another meringue layer, again peeling off the parchment, and on it spread ⅓ of the ganache. Top with a third layer of meringue and spread with the remaining half of the buttercream, then place the final layer of meringue on top. Spread the remaining ²/³ of the ganache in an even layer over the top and two long sides of the cake. Press the chopped peanut brittle or additional chopped peanuts onto the two long sides of the cake. You will not use all the brittle; save the remainder to scatter on each plate (or as a treat for the chef). Spoon the reserved buttercream into the pastry bag and pipe a line of shells down the top edges of the long sides. Refrigerate for at least 1 hour and up to 24 hours.
- While the cake is cold, trim the two short ends with a serrated knife to neaten the appearance and show off the layers inside. Let the cake sit at room temperature for at least 40 minutes before serving. Cut the cake with the serrated knife into 1-inch-thick slices.
Storing: The cake is at its best within 24 hours. It can be stored, plastic wrap pressed against the cut edges, in the refrigerator for up to 4 days, but the meringue will soften considerably from the moisture in the refrigerator and the fillings—and the peanut brittle will soften and begin to melt. Return to room temperature before serving.