Melt the chopped chocolate and butter together in a double boiler or microwave. Stir well and let cool slightly. Beat the egg yolks with half of the sugar and the salt until the mixture is pale and fluffy.

Add the melted chocolate and butter mixture in three or four additions, mixing well each time. In a blender, grind the hazelnuts into a fine meal and add them to the batter.

Beat the egg whites with the remaining sugar until stiff peaks form. Using a spatula, gently fold the meringue into the chocolate mixture in three additions, using upward, gentle motions.

Pour the batter into a 6- to 8-inch (15-20 cm) springform pan lined with parchment paper. Bake in a preheated conventional oven at 350°F (180°C) for about 35 minutes, or until the cake is set but still moist in the center.

Let the cake cool slightly in the pan, then remove and let cool completely. Before serving, dust the hazelnut Caprese cake with powdered sugar.