For the dough, wash oranges hot, dry them and finely grate the peel. Squeeze 1 orange and measure 100 ml juice. Grease a box form (30 cm long; approx. 2 l capacity) and dust with flour.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
Cream 200 g butter, sugar, vanilla sugar and orange zest with the whisks of the mixer. Stir in the eggs one by one. Mix 350 g flour, 1 pinch of salt and baking powder and stir in alternately with yoghurt and 100 ml orange juice.
Pour the dough into the tin and bake in a hot oven for 50-60 minutes. After about half the baking time, cut the surface of the cake lengthwise and make a groove with a wooden spoon. Repeat this procedure once or twice during the remaining baking time.
Remove the finished cake and let it cool in the mould for about 10 minutes. Then turn out onto a cake rack and let it cool down.
Spread the jam in the groove of the crust and on the cake. Chop the chocolate and melt it in a hot water bath, then pour it in strips over the cake and let it dry.