Melt the butter and let it cool down again. Mix 600 g flour, sugar, lemon peel, baking powder and salt. Add eggs and butter and knead into crumbles. Put 1/3 of the crumbles in the fridge for about 30 minutes. Spread the remaining crumbles on a greased baking tray (39 x 32 cm; photo tray is 18 x 35 cm; recipe for this is exactly half; baking time remains the same). Press to a smooth base with your hands.
Spread the plum puree and jam in patches. Loosely mix cold crumbles and 2-3 tablespoons of flour and sprinkle over it in a grid pattern. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) on the lower rack for approx. 20 minutes. Bake on the middle shelf for another 15 minutes. Let the cake cool down and dust with icing sugar