Let mandarin oranges drain very well in a sieve. Put the sour cream in a bowl. Wash the sour cream cup, dry well and use it as a measuring cup. Grease a box form (approx. 2 3/4 l capacity; 30 cm long) and dust with flour.
Preheat the oven (electric cooker: 175°C/circulating air: 150°C/gas: see manufacturer).
Beat the eggs, sugar and vanilla sugar with the whisk of the mixer until thick and creamy. Stir in bionade, oil and sour cream. Mix flour and baking powder and stir in portions briefly. Fold the mandarins into the dough.
Fill the dough into the mould and smooth it down. Bake in a hot oven for about 1 1/4 hours. If necessary, cover with aluminium foil after about 50 minutes baking time. 5 minutes before the end of the baking time, check with a wooden skewer whether the dough is baked through.
If there is still tough dough sticking to the spit, continue baking the cake for 5-10 minutes (no longer, otherwise it will become dry).
Take the cake out of the oven and let it cool in the mould for about 15 minutes. Then carefully remove the cake from the edge with a knife and let it cool down in the mould. Turn the cake out of the mould and dust with icing sugar.