Coarsely chop 100 g chocolate and melt in a hot water bath. Grease a box mould (25 cm long; approx. 1.7 l capacity) and dust with flour. Preheat the oven (electric cooker: 180 °C/recirculating air: 160 °C/gas: see manufacturer).
Mix 300 g flour, baking powder, baking soda, sugar, vanilla sugar and 1 pinch of salt. Peel 2 bananas, puree the pulp with 100 ml oil and yoghurt. Add banana puree and eggs to the flour mixture, stir in briefly.
Fill about half of the dough into the form, smooth it down. Pour half of the liquid chocolate on top. Then add the remaining dough and finally the remaining liquid chocolate. Pull a fork spirally through the dough. Bake in a hot oven for 45-50 minutes. Remove from the oven, leave to rest in the mould for about 10 minutes, then turn over. Let the cake cool down on a cake rack.
Roughly chop 200 g chocolate and melt in a hot water bath with 2 tbsp. oil. If necessary, cut the cake surface a little straighter. Peel 2 bananas, cut in half lengthwise and spread a thin layer of lemon juice on them. Place lengthwise on the cake. Cover with chocolate and leave to dry. Whipped cream tastes good with it.