Wash the lemon, grate dry and thinly grate the peel. Squeeze the lemon.
Separate two eggs. Put both yolks aside for later. Beat the 2 egg whites with 2 tablespoons of cold water until stiff, adding 120 g sugar.
Separate 1 egg again. Stir this egg and the two yolks placed to one side into the mixture from step 2. Put the egg whites aside this time.
Mix flour, starch and baking powder. Sift the flour-mix into 2 portions over the mixture and fold in. Pour the mixture into a greased springform pan (26 cm Ø).
Separate two more eggs. The egg yolks are not processed further. Beat both egg whites together with one of them aside and 1 teaspoon of lemon juice until stiff, pouring in 120 g of sugar. Carefully pour the meringue mixture onto the sponge cake, leaving at least 1 cm of edge free. Bake the sponge in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 30-35 minutes. As soon as the meringue is golden brown, slide a tray into the top baking tray. Remove the sponge cake from the oven, remove from the rim and allow to cool.
Soak gelatine in cold water. Mix sour cream, 75 g sugar, lemon juice and zest and liqueur. Squeeze the gelatine and dissolve in a small pot. Stir in 3 tablespoons of sour cream, then stir into the remaining cream. Chill the cream until it starts to gel. Whip cream until stiff and fold into the cream.
Cut the sponge in half horizontally so that the lower half is slightly thinner than the upper half. Place a cake ring around the lower cake base. Spread the cream on top and cover with the other half. Chill the cake for at least 3 hours, then dust with icing sugar.