Preheat oven to 175°C (155°C fan-forced). Grease and line a 1L loaf tin with baking paper, leaving roughly 2cm hanging over the edge.
Sift the flour and baking powder, and combine in a bowl with the cinnamon, brown sugar, walnuts, and a pinch of salt.
Mash 3 of the bananas in another bowl, then add the eggs, vanilla, milk, and melted butter and mix to combine. Slowly fold the banana mixture into the dry ingredients, taking care not to over mix.
Pour the batter into your loaf tin and slice the remaining banana to garnish. Put into the oven for an hour, or until a skewer through the centre comes out clean. Remove from the oven and rest for 10 minutes before transferring from the pan to a cooling rack. Once cooled, simply slice and enjoy!