Combine the hazelnuts, baking powder and egg yolks. Beat the egg whites, water, sugar and vanilla sugar until stiff. Add the hazelnuts to the beaten egg whites and gently fold in. Grease a springform pan well and sprinkle with breadcrumbs. Pour the batter into the pan and bake at 170°C for 60 minutes.
Prepare a pudding using the pudding powder according to the package instructions, but with 100 g of sugar. Allow the pudding to cool to room temperature, stirring frequently to prevent a skin from forming.
Place the room temperature softened butter in a bowl. Stir the pudding into the butter very slowly (by teaspoonfuls at first) with a mixer.
Divide the cooled cake into three layers. Divide the buttercream into thirds as well. Put the first part of the buttercream on the first cake layer.
Place the second cake layer on top and spread the next part of buttercream. Place the third cake layer on top and spread the remaining buttercream over the entire cake.
If you have some cream left over, you can decorate the cake with the help of a piping bag. Finally, garnish with the whole hazelnuts.