Roughly chop the nuts. Mash bananas with a fork. Mix flour, vanilla sugar, baking powder and salt in a bowl. Add bananas, sugar, eggs, oil and buttermilk bit by bit and mix with the whisk of the hand mixer to a smooth dough.
Stir in the nuts.
Fill the dough into a greased and floured box form (approx. 11 x 30 cm) and smooth it down. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 50-55 minutes (stick test).
Take the cake out of the oven, let it cool down, turn it over after about 20 minutes. Let the cake cool down on a cake rack. Dust with icing sugar if necessary.