Mix flour, sugar and salt.
Add butter in small pieces and work into crumbles.
Put about ⅔ of the crumble into a Ø26cm springform pan, press and spread out with your fingers and a spoon. (It is best to form the edge first and then the base).
Mix the cinnamon into the remaining crumble.
Put the springform pan and the remaining crumble in the fridge.
Mix the custard powder with sugar and a bit of the milk until smooth.
Bring the remaining milk to boil and stir in the custard powder.
Leave to simmer briefly and then remove from the heat.
Cut the plums in half and remove the seeds.
Melt the butter in a pan.
Add the plums.
Add the sugar and cornflour then mix.
Let the plums steam briefly while stirring. (Not too long, they don't have to become soft).
Put to the side.
Take the springform pan out of the fridge.
Pour the custard on top and spread evenly.
Spread the plum filling on top.
Finally, spread the crumble over the cake with your fingers. (You can decide the size of the crumble by kneading it in your hand).
Bake in the preheated oven at 180° C for about 50 minutes.
Leave to cool and sprinkle with icing sugar.