Put 500 g flour, egg, 60 g sugar and grated lemon rind in a bowl. Mix 15 g sugar and yeast. Melt 50 g fat, add cold milk (lukewarm mixture) and add to the flour together with the yeast. Knead with the dough hooks of the hand mixer to a smooth yeast dough. Cover and leave to rise in a warm place for about 30 minutes.
For the crumble dough, mix 250 g flour, 125 g sugar and vanillin sugar in a bowl. Melt 175 g fat and pour hot into the flour mixture. Knead into crumbles with the dough hooks of the hand mixer. Coarsely chop the walnuts and mix in. Wash the plums, drain well and cut them half lengthwise. Remove stones. Grease a fat pan (approx. 32 x 38 cm) well and dust with flour. Roll out the yeast dough and let it rise again in a warm place for about 20 minutes. Place plums on the dough and sprinkle with walnut crumbles. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 40-45 minutes.
Remove stones. Grease a fat pan (approx. 32 x 38 cm) well and dust with flour. Roll out the yeast dough and let it rise again in a warm place for about 20 minutes. Place plums on the dough and sprinkle with walnut crumbles. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 40-45 minutes. Let it cool down and cut it into approx. 24 pieces. Serve with whipped cream