Wash the potatoes and cook with the skin in boiling water for about 20 minutes. Drain, rinse with cold water and peel. Let the potatoes cool down. Separate eggs. Beat egg yolk and sugar for about 15 minutes until white-creamy.
Press the potatoes through a potato ricer. Beat 3 egg whites until very stiff. Mix flour, baking powder, hazelnuts, 1 teaspoon cinnamon and lemon peel and stir into the egg yolk mixture. Fold in potatoes and beaten egg white.
Pour the mixture into a greased and breadcrumbed springform pan (24 cm Ø), smooth it down and bake in a preheated oven (electric: 200 °C/ gas: level 3) for 50-60 minutes. Remove from the baking tin and let it cool down.
Cut through once, spread the lower bottom with the jam and put back together again. Stir icing sugar with the remaining egg white and lemon juice until smooth. Spread the cake with it. Coarsely chop the hazelnut kernels and sprinkle over the still moist icing.
Dust with the remaining cinnamon. Results in about 12 pieces.