Preheat the oven to 350°F.
Grease a 9-inch springform pan and line the bottom with parchment paper.
In a double boiler or heatproof bowl set over a saucepan of simmering water, melt the chocolate and butter together, stirring occasionally, until smooth and combined.
Remove the bowl from the heat and stir in the toasted hazelnuts.
In a stand mixer or with a hand mixer, beat the egg yolks, sugar, and salt together until pale and thick.
Fold the chocolate mixture into the egg mixture until well combined.
In a separate bowl, beat the egg whites until stiff peaks form.
Gently fold the egg whites into the chocolate mixture until no white streaks remain.
Pour the batter into the prepared pan and smooth the top.
Bake for 35-40 minutes or until the cake is set and a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for 10 minutes before removing the sides and transferring it to a wire rack to cool completely.
Serve the cake with whipped cream and chopped hazelnuts for garnish.