Roughly chop the chocolate. Whisk butter, vanilla sugar, salt and sugar with the whisk of the hand mixer until foamy. Stir in the eggs one after the other. Stir in 100 g yoghurt. Mix flour, starch and baking powder and stir in briefly (not too long). Fold in chocolate. Pour the dough into a greased, floured box form (25 cm long), smooth it down and bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 50-60 minutes.
Remove from the oven, place on a rack and let cool down a little. Turn out of the mould and let cool down. Stir the groats and flash-cast with a fork for about 1 minute until the groats are slightly moist. Spread evenly on the cake. Stir 300 g yoghurt smooth with a fork. Pour into a piping bag, cut off the tip and spread the yoghurt evenly over the cake
2 1/2 hours waiting time