For the crumbles, melt 175 g butter. Coarsely chop the walnuts. Mix 250 g flour, 125 g brown sugar, 2 tsp cinnamon, 1 pinch of salt and half the walnuts in a bowl. Add hot butter all at once.
Knead into crumbles with the dough hooks of the mixer. Leave to cool.
Grease a fat pan (deep baking tray; 32 x 38 cm) and dust with flour. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Mix 250 g butter, 200 g brown sugar, 2 teaspoons cinnamon, vanilla flavouring, lemon peel and 1 pinch of salt with the whisks of the mixer for about 5 minutes until creamy.
Stir in the eggs one after the other, alternating with 1 tablespoon of flour. Peel the bananas, crush them finely with a fork and stir into the egg-fat mixture together with the yoghurt. Mix 200 g flour and baking powder, then briefly stir in the remaining nuts.
Spread the dough in the fat pan and sprinkle with crumbles. Bake in a hot oven for 30-35 minutes (when you are ready, the cake may still be a little moist!). Let the cake cool down in the fat pan.
For the caramel sauce, caramelise white sugar in a wide pot. Take the pot off the stove and stir in 1 tbsp. butter. Stir in the double cream well. Let it cool down, stirring in between if necessary.
Cut cake into pieces, add caramel sauce.