Warm up the buttermilk. Mix the flours in a mixing bowl. Press a depression in the middle. Crumble the yeast into the hollow. Mix with sugar and 6 tablespoons of buttermilk and cover with some flour from the rim. Cover and leave to rise for about 10 minutes. Add remaining buttermilk and walnuts.
Use the dough hooks of the hand mixer to work everything into a smooth dough and let it rise for another 30 minutes, covered. In the meantime peel the bananas and cut them into cubes. Add banana cubes to the dough and knead vigorously on a generously floured work surface. Form 4 baguettes, cut them diagonally and place them on a baking tray lined with baking paper. Dust with a little flour and leave to rise for a further 30 minutes. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 35 minutes.