Wash the plums, drain, halve and stone them. Separate eggs. Beat the fat and sugar with the whisk of the hand mixer until foamy. Stir in egg yolks one after the other. Add lemon zest and salt.
Alternately stir in flour and 5 tablespoons of milk. Beat the egg white until stiff and fold into the dough. Put the dough into a greased and breadcrumbed springform pan (26 cm Ø) and smooth it down. Place the halved plums on the dough with the curvature upwards. Heat the jelly in a pot and spread it on the plums. Bake in a preheated oven (electric: 175 °C/ gas: level 2) for approx. 60 minutes. Let it cool down on a cake rack. For the sauce, put the remaining milk in a tall cup and add sauce powder while stirring. Stir vigorously for about 1 minute.
Bake in a preheated oven (electric: 175 °C/ gas: level 2) for approx. 60 minutes. Let it cool down on a cake rack. For the sauce, put the remaining milk in a tall cup and add sauce powder while stirring. Stir vigorously for about 1 minute. Arrange the cake on a plate and dust with icing sugar. Add the vanilla sauce in a small pot