Drain the plums and collect the juice. Stir four tablespoons of juice and starch until smooth. Boil up the rest of the juice and two tablespoons of sugar. Add lemon juice and liqueur. Stir in cornflour and bring to the boil again.
Fold in the plums and fill into an ovenproof dish. Beat the egg yolks, a tablespoon of water, vanillin sugar and lemon zest, stir in the flour. Beat the egg whites until stiff, add the remaining sugar.
Carefully fold the egg yolk mixture into the beaten egg white. Cut off large dumplings and place them on the compote. Bake in the preheated oven (electric cooker: 175 °C/ gas: level 2) for twelve to 15 minutes, dust with icing sugar and serve immediately.