Preheat the oven to 350°F (180°C) and spray a 9x5 loaf pan with non-stick spray and line the bottom with parchment paper. Set aside.
Sift all purpose flour into a mixing bowl and add sugar, baking soda, salt and cinnamon and mix together.
Add the butter to a microwave safe bowl and microwave until melted. Mash the bananas and measure them properly, and then add the mashed banana to the melted butter along with the eggs and vanilla and mash everything together until well mixed.
Add the wet ingredients in with the dry and mix it into a thick batter.
Add the walnuts if using and fold them into the batter.
Transfer the batter to your prepared loaf pan and smooth down the top with the back of a spoon.
Add a few more chopped walnuts to the top of the banana bread.
Place into the oven to bake for 60 minutes. You can bring it out at the 40 minute mark and tent with foil to prevent it over-browning. Tenting with foil is just placing the foil over the top in a tent shape so that it doesn't press down on the banana bread from the top. Place it back into the oven for the remaining 20 minutes. It's cooked when a toothpick inserted into the center of the banana bread comes out clean.
Let it cool in the pan for a few minutes and then transfer to a wire cooling rack to cool completely before slicing and serving.