Melt 30 g butter. Beat eggs, vanilla sugar, salt and sugar with the whisk of the hand mixer until foamy. Stir in flour and milk, then add the liquid butter.
Spread the plums on the bottom of a greased and floured closed baking dish (approx. 24 cm Ø; not a springform pan!). Carefully pour the dough over it. Cake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see
manufacturer) bake for about 1 1/2 hours. Cover the cake with aluminium foil after about half the baking time.
About 5 minutes before the end of the baking time, put 20 g butter in flakes on the cake and bake until done. Remove. Leave to cool for about 20 minutes. Carefully turn the cake over and let it cool down on a cake rack.