- 10 tablespoons unsalted butter, plus more for pan
- 1 cup hazelnuts
- All-purpose flour, for pan
- 3/4 cup firmly packed light-brown sugar
- 6 large whole eggs, separated
- 4 large egg whites
- 12 ounces bittersweet chocolate, melted and cooled, plus 4 ounces roughly chopped
- 1 tablespoon pure vanilla extract
- 1 tablespoon rum, optional
- Pinch of salt
- 1 tablespoon cornstarch
- 1/4 teaspoon cream of tartar
- 1 cup superfine sugar
- Preheat oven to 350 degrees. Butter a 9-by-3-inch springform pan. Line bottom with parchment. Butter parchment, and sprinkle with flour; tap out excess. Set aside.
- Spread hazelnuts in a single layer on a rimmed baking sheet. Toast until fragrant and skins start to crack, 10 to 15 minutes. Remove from oven; rub vigorously with a clean kitchen towel to take off skins. Let cool; roughly chop. Set aside.
- Make cake batter: In the bowl of an electric mixer fitted with the paddle attachment, cream butter and brown sugar until pale and smooth. Add egg yolks, one at a time, beating well after each addition, until mixture is light and fluffy. Add melted chocolate, vanilla, and rum, if using; beat until combined. Set aside.
- In a clean mixer bowl, combine 6 egg whites and salt; using the whisk attachment, beat on high speed until soft peaks form, about 2 minutes. Stir one-third of the egg whites into chocolate mixture. Fold in remaining beaten egg whites just until combined. Pour batter into prepared pan, and bake 25 minutes.
- Meanwhile, make meringue: Combine hazelnuts, chopped chocolate, and cornstarch in a small bowl, and set aside. Place remaining 4 egg whites and cream of tartar in a clean mixer bowl; using a clean whisk attachment, beat on high speed until frothy. With mixer running, slowly add superfine sugar; continue beating until stiff peaks form, about 8 minutes. Fold in hazelnut mixture.
- Remove cake from oven. Using an offset spatula, spread meringue mixture on top of cake, and return to oven. Bake until meringue is lightly browned and crisp, 25 to 30 minutes. Transfer pan to a wire rack; let stand 10 minutes. Run a knife around the edge of the cake to loosen, and release sides of pan. Let cool, about 30 minutes, before slicing and serving.
This cake is just as delicious without the meringue topping; bake the chocolate until it is crackling and slightly firm, 40 to 45 minutes.