Separate the eggs for the sponge cake. Beat the egg yolks and 2 tablespoons of warm water until frothy. Add 60 g sugar and lemon zest and stir until creamy. Beat the egg whites until stiff and add the remaining sugar.
Fold into the egg yolk mixture. Mix flour, starch and baking powder, sieve over it and carefully fold in. Spread the sponge mixture on a baking tray lined with baking paper and bake in a preheated oven (electric range: 200 °C/ gas: level 3) for approx. 10 minutes.
Take out and turn onto a tea towel sprinkled with sugar. Remove the baking paper. Let the sponge cake cool down and cut it horizontally once. Spread the lower half with plum jam and place the upper half on top.
Wash 500 g plums, cut in half, stone and sauté in lemon juice. Spread on the sponge cake with the cut surface facing down. For the cream, whip cream until stiff. Whip the cake aid and 400 ml water for about 2 minutes until frothy.
Stir in yoghurt and fold in cream with a whisk. Spread the cream on the plums until smooth. Chill for about 1 hour. Cut the cake into 15 pieces (approx. 3x5 cm). To decorate, wash the remaining plums, remove stones and cut into flat slices.
Mix the cake glaze, 200 ml water and prune juice. Bring to the boil while stirring. Add plum slices, spread lukewarm on the slices and let cool.