The cake can be made a day ahead. Wrap in plastic wrap and store at room temperature.
5 1/2 ounces (1 1/3 cups) hazelnuts, toasted and skinned
5 1/2 ounces bittersweet chocolate, finely chopped
Grated zest from 1 lemon
Grated zest from1 orange
6 large eggs, separated
1/2 cup sugar
Ice cream to serve the cake with (I like coffee, caramel or hazelnut)
Preheat the oven to 350 °.
Line the bottom of a 9-inch round cake pan with parchment paper.
Put the hazelnuts, chocolate, and lemon and orange zests in a food processor fitted with a metal blade. Finely grind the mixture using on/off pulses. Set aside.
In the bowl of an electric mixer with the whisk attachment, whip the egg yolks and 1/4 cup of the sugar on high speed for about 2 minutes, until slightly increased in volume. Decrease to low speed and stir in the ground nut mixture. The batter will be very stiff.
Whip the egg whites in a clean bowl of an electric mixer on medium speed until frothy. Increase to high speed and whip the whites until soft peaks form about 1 minute. Decrease to medium speed and add the remaining 1/4 cup sugar in a steady stream. Once it has all been added, increase to high speed and whip until soft peaks form. Fold one-half off the egg whites into the batter. When they are almost completely incorporated, fold in the remaining egg whites. Gently spread the batter into the prepared pan. Bake the cake until a skewer inserted in the middle comes out clean, about 25 minutes.
Cool the cake to room temperature. Run a knife around the inside edge of the pan. Remove the cake from the pan by placing a large plate or platter on top of the cake pan. Invert the pan and the plate together. Remove the pan and carefully remove the parchment paper.