Grease a box mould (30 cm long; approx. 2 l capacity) and dust with flour. Coarsely chop the walnuts. Peel bananas. Save a banana peel as a template for decoration. Chop the bananas coarsely, then mash them finely with a fork to make puree.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Cream 175 g butter, sugar, vanilla sugar and 1 pinch of salt with the whisks of the mixer. Stir in the eggs one by one.
Mix 350 g flour, poppy seeds, walnuts and baking powder. Alternately stir in flour mix, banana puree and crème fraiche.
Fill the dough into the box form and smooth it down. Bake in a hot oven for 50-60 minutes. After approx. 50 minutes, check with a long wooden skewer whether the dough is baked through. If there is still tough dough sticking to the skewer, bake the cake for another 5-10 minutes.
(No longer, or the cake will dry.).
Remove from the oven and leave to cool in the mould for about 1 hour. Turn out of the mould and let it cool down. Dust the cake with icing sugar and decorate with the marzipan-banana peel if desired.