Wash the plums, drain, halve and stone them. Cream 125 g fat, 100 g sugar and vanilla sugar with the whisk of the hand mixer. Stir in the eggs one after the other. Stir in chestnut purée.
Mix flour, baking powder and cinnamon and stir in alternately with the milk.
Fill about half of the dough into a greased, flour-pocketed box form (11 x 30 cm; 7 cm high). Place about half of the plums on the dough, close together. Spread the rest of the dough on the plums and cover with the remaining fruit.
Bake in the preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see manufacturer) for about 30 minutes.
Meanwhile, bring 100 g fat, 100 g sugar and 2 tablespoons milk to the boil in a saucepan. Let simmer for about 5 minutes at low heat, stir in cornflakes. Cover the cake with the cornflakes mixture and continue to bake at the same temperature for about 20 minutes.
Take out and let cool on a cake rack.
Carefully remove the cake from the mould and place it on a cake plate. Roughly chop the chocolate and melt it over a warm water bath. Decorate the cake with chocolate.