Clean, wash, halve and stone the plums. Mix 200 g flour, 100 g butter, 50 g sugar and ground walnuts into crumbles using the dough hooks of the hand mixer. Mix 150 g sugar and 100 g butter with the whisk of the hand mixer. Add rum. Stir in eggs one after the other. Mix 150 g flour, baking powder, cocoa and ground almonds. Alternately stir in mineral water.
Spread the dough on a greased baking tray. Spread the plums on top with the skin side down. Crumble the crumble over it. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 30-35 minutes. Leave to cool. Stir icing sugar and cherry nectar until smooth and decorate the cake with it