Start by preparing the buttermilk by mixing the milk and lemon juice in a bowl.
Preheat your oven to 180°C.Butter your cake mold on all surfaces. Without a salad bowl, mix the flour, yeast, baking soda, cinnamon and salt.Add the roughly chopped walnuts. In another bowl, beat the butter and sugar until creamy.Add the eggs one by one and mix each time.Puree 3 bananas using a fork and add them to the mixture.Finally pour in the buttermilk and mix. Gradually add the dry ingredients from the other bowl, stirring until well incorporated.
Pour into your buttered cake mold.Peel and cut the last banana in 2 then place it on the dough. It's optional but I think it looks good. Bake for 50 to 55 minutes.To check the cooking, insert the blade of a knife inside the cake, it should come out without any dough on the blade. Remove from the oven and let cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.
