Wash plums, cut in half, stone. Grease a fat pan (approx. 32 x 39 cm; at least 3.5 cm deep) and dust with flour. Cream butter, sugar, vanilla sugar and 1 pinch of salt. Stir in the eggs one by one.
Mix flour, sauce powder and baking powder and stir in briefly.
Spread the dough on the fat pan. Spread the plums on top. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for approx. 50 minutes (cover with aluminium foil after approx. 40 minutes if necessary).
Sprinkle the cake with sugar crystals and let it cool down.