Roughly chop the walnuts. Mix eggs, yoghurt, cream and 300 g sugar with the whisk of the hand mixer. Mix flour, baking powder and nuts, stir into the egg mixture
Put the dough into a greased, flour-spread fat pan (32 x 39 cm) of the oven and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 30 minutes. Take the finished cake out of the oven and let it cool down
Briefly whip the sour cream, 150 g sugar and vanilla sugar with the whisks of the hand mixer. Spread the cream over the cake and spread evenly. Refrigerate the cake for at least 4 hours or better 2 hours
Wash the plums, cut in half, remove stones. Wash oranges hot, grate dry, grate peel thinly. Halve oranges, squeeze juice. Boil up liqueur, 4 tablespoons sugar and orange juice. Add plums, bring to the boil and stew for about 10 minutes. Stir starch and 4 tbsp. water until smooth. Pour into the boiling liquid while stirring, bring to the boil again and simmer for 1-2 minutes. Let the plums cool down
Cut the cake into pieces and spread the plums on top
Waiting time approx. 5 hours