- 1 cup hazelnuts, divided
- 1/2 cup (1 stick) unsalted butter, cubed, plus more for greasing
- 7 oz dark chocolate, broken into squares
- 6 eggs, separated
- 1 cup packed brown sugar
- 3 Tbsp hazelnut liqueur or amaretto liqueur
- 6 oz chocolate and hazelnut spread
Preheat oven to 400°.
In a frying pan over medium-high heat, add hazelnuts and toast, tossing occasionally, until nutty smelling and skins are beginning to fall off, about 5 minutes. Transfer nuts to clean dish towel, wrap up nuts, and let steam for 1 minute. Rub nuts with towel to remove as many skins as possible. Let cool. In a food processor fitted with the blade attachment, add 3/4 cup hazelnuts and pulse until finely ground. Set aside.
Reduce oven temperature to 350°. Butter and flour a 9″ springform cake pan and line base with parchment paper.
In a heatproof bowl set over a saucepan of gently simmering water, add butter and chocolate. Let melt, stirring occasionally. Add hazelnut meal, stir to combine, and let cool.
In a bowl, add egg yolks and brown sugar and, using a hand-held blender, beat until pale and creamy, about 5 minutes. Add chocolate mixture and hazelnut liqueur and stir to combine. Wash and dry beaters well.
In a clean bowl, whisk egg whites until stiff peaks form. Add a dollop of egg whites to chocolate mixture and fold until streaks appear. Continue to add egg whites, folding to combine, until all whites have been incorporated into chocolate mixture.
Pour batter into prepared pan and bake until cake edges are dry and a toothpick inserted into center of cake comes out clean, 35-40 minutes.
Remove cake from oven and let cool in pan for 30 minutes. Undo springclip and invert cake on to a serving plate, then peel off parchment paper and let it cool completely.
When cake is cold, spread it with chocolate and hazelnut spread and sprinkle with remaining 1/4 cup hazelnuts.