01 - Heat the oven to 350°F. Grease a 9-inch springform pan and line the base with parchment paper.
02 - Melt the dark chocolate and butter together in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and let cool slightly.
03 - Stir in optional espresso powder, vanilla extract, and salt into the melted chocolate mixture.
04 - In a large bowl, whisk the egg yolks and granulated sugar until pale and thick.
05 - Fold the cooled chocolate mixture into the egg yolk batter until incorporated.
06 - Gently fold the ground toasted hazelnuts into the combined batter.
07 - Whip the egg whites to stiff peaks in a clean bowl. Fold one-third into the batter to loosen, then gently fold in the remaining whites until just combined.
08 - Pour the batter into the prepared springform pan and smooth the surface. Bake for 30 to 35 minutes until the edges are set and the center remains slightly soft.
09 - Allow the torte to cool completely in the pan. Remove the ring, transfer to a serving plate, dust with powdered sugar, and garnish with toasted hazelnuts. Serve optionally with whipped cream or crème fraîche.