Stir butter with 100 g of sugar until foamy. Gradually stir in the egg yolk, then add the remaining ingredients such as buckwheat flour, food starch, grated nuts, sifted baking powder and lemon zest. Now fold in the whipped egg whites with the remaining 100 g of sugar. Add the mass to the cake pan and bake in the preheated oven (at 180°C for about 40 minutes).
Let the baked cake cool and cut in half, so that you get two round halves.
Mix mascarpone with the whipped cream and garnish on a half cake with fresh seasonal fruits. Put on the other half and chill for 20 minutes. Then serve.