Mix flour, sugar and salt.
Cut butter in small pieces and crumble in the flour.
Add an egg and knead to a dough.
Grease and flour a 26 cm diameter springform.
Use about ⅔ of the dough, roll out and lay out the springform. The edge should be about 3 cm high.
Place prepared springform and the rest of the dough in the fridge.
Mix cold milk, custard powder, sugar, and poppy seeds in a pot.
Bring to boil, stirring constantly till mixture thickens.
Remove from heat, leave to cool for 10 minutes.
Spread filling in prepared spingform with shortcrust.
Pit plums and cut in quarters and spread over poppyseed custard filling.
Mix brown sugar and cinnamon and sprinkle on plums.
Crumble the rest of shortcrust pastry and spread over cake.
Bake in preheated oven at 180° C for 40 to 45 minutes.
Leave to cool.