Preheat the oven to 165 degrees C. Grease and flour a 23cm springform cake tin.
Grind hazelnuts until very fine. Add baking powder and set aside.
In a large bowl whip the egg yolks with the sugar until pale yellow in colour. Beat in the ground hazelnut mixture.
In a separate clean bowl, with a clean whisk, whip the egg whites until stiff. Quickly fold 1/3 of the egg whites into the yolk mixture then add the remaining whites and fold in until no streaks remain.
Pour into the springform cake tin. Bake in preheated oven for 60 to 75 minutes or until top of cake springs back when lightly tapped. Cool on wire rack.
When the cake is cool slice horizontally into 3 layers. Whip the cream until stiff and spread generously between layers, on top and on the sides of the cake. Sprinkle chopped hazelnuts on top for decoration.