Preheat the oven to 325°F (165°C). Grease and flour a 9 inch springform pan. 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 color. 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 a 9 inch springform. Bake in preheated oven for 60 to 75 minutes, or until top of cake springs back when lightly tapped. Cool on wire rack.
When 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.