Put flour and salt in a bowl. Stir yeast with 1 tablespoon of sugar until liquid. Add remaining sugar and liquid yeast to the flour. Melt fat and add milk. Add lukewarm milk-fat mixture and egg to the flour and knead all ingredients to a smooth dough.
Cover and leave to rise in a warm place for about 20 minutes. Wash, quarter and stone the plums. Knead the dough well again. Grease the fat pan of the oven (37x32 cm) and roll out the dough.
Spread the plums on the dough with the open side facing upwards. Mix 50 g of sugar crystals with the cinnamon, sprinkle over it and let it stand in a warm place for another 15 minutes. Bake in the preheated oven (electric: 175 °C/ gas: level 2) for 35-40 minutes.
Leave the cake to cool, sprinkle with the remaining decorating sugar and serve. Makes about 20 pieces.