Grease a box cake tin (25 cm long, 1.5 l capacity) and dust with flour. Wash, halve and stone the plums. Preheat the oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see
Mix flour, 125 g sugar, vanilla sugar, baking powder and 1 pinch of salt. Add eggs and both yoghurts. Mix with the whisks of the mixer to a smooth dough.
Fill the dough into the mould. Spread the plums evenly on top. Mix oat flakes, 1 tablespoon sugar, cinnamon and oil. Spread on the dough and bake in a hot oven for 50-60 minutes. If necessary, cover the cake with aluminium foil about 20 minutes before the end of the baking time.
Take the cake out of the oven and let it cool in the mould on a cake rack for about 40 minutes. Turn the cake out of the tin and let it cool down. Whipped cream tastes good with it.