For the plum tiramisu, soak and squeeze the gelatine according to the package instructions.
Bring the milk to the boil and stir in the sugar. Add the gelatine to the milk-sugar mixture and add the lemon juice. Prepare the vanilla pudding in the milk mixture, bring to the boil and then allow to cool. Beat the egg whites until firm and fold into the milk pudding mixture.
Peel the plums, remove the seeds and cut them into small pieces. Mix the yogurt and curd with the plums and then fold into the custard. Line a container with biscuits soaked in plum liqueur and spread a layer of plum custard. Then alternate layers of biscotti and plum custard. Finish with a layer of plum custard and smooth. Refrigerate the finished plum tiramisu for at least 4 hours. Before serving, sprinkle with sugar, cocoa or cinnamon according to taste.