Preheat your oven to 350F/175C.
Place 4 peeled bananas in a bowl and mash with a fork. Set aside.
In another bowl, combine softened butter, white sugar, brown sugar, and white miso. Mix with a handheld mixer or stand mixer until light and fluffy, about 5 minutes. Add mashed bananas, eggs, and vanilla extract. Mix for 30 seconds until combined.
Sift the all purpose flour, baking powder, cinnamon, and nutmeg. Transfer into the bowl with the banana mixture. Gently fold until well combined. Be sure not to overmix!
Spray all sides of the loaf tin with non-stick cooking spray. Pour the batter into the tin and spread the top so it's an even layer.
Cut the last banana in half and place it on top of the batter.
Bake for 60 to 90 minutes. Make sure to check for doneness at the 60 minute mark by sticking a toothpick in the center. If it comes out clean, it's ready! If it comes out with crumbs or sticky batter, it needs more time. Keep checking for doneness every 10 minutes until it's ready.
Allow to cool for at least 30 minutes. Serve and enjoy!