- 1 stick (8 tablespoons) unsalted butter, plus more for the baking pan
- All-purpose flour, for the baking pan
- 4 ounces bittersweet chocolate, chopped
- ¾ cup skinned hazelnuts, lightly toasted
- 6 large eggs, separated
- 2 tablespoons sugar
- One 13-ounce jar chocolate-hazelnut spread, such as Nutella, at room temperature
- Pinch of kosher salt
- 1 tablespoon brandy
Preheat the oven to 350 degrees. Butter and flour a 9-inch springform pan. Melt the chocolate in the top of a double boiler or a metal bowl set over a saucepan of simmering water. Let it cool slightly. Grind the hazelnuts in a food processor until they are fine but not pasty.
In a mixer fitted with the whisk attachment, beat the egg whites until they are foamy. Add the sugar, and beat until the whites form stiff peaks, about 2 minutes. In a clean bowl, with the paddle attachment, beat the butter and chocolate-hazelnut spread until light, about 2 minutes. Add the yolks, salt, and brandy, and mix until smooth. Add the melted chocolate and ground hazelnuts, and mix until smooth. Remove the bowl from the mixer.
Stir about a quarter of the egg whites into the chocolate mixture to lighten it, and then gently fold in the remaining egg whites. Don’t overmix. Pour the mixture into the prepared pan. Bake until a tester comes out clean, about 45 to 50 minutes. Let the cake cool for about 10 minutes, then open the spring and remove the side ring. Let the cake cool thoroughly before serving. Slide a broad metal spatula, or two, under the cake to separate it from the metal pan bottom, then lift and set the cake on a serving plate.