Preheat the oven to 350°F and grease a 9 x 5 loaf pan with butter.
In a medium bowl, combine the whole wheat flour, baking soda, salt and cinnamon, and stir to combine.
In a small bowl, add the ripe bananas. Using the back of a fork, mash them until they are well combined, set aside.
In a large bowl, add the sugar and melted butter and whisk together until well combined. Add the eggs and vanilla extract and whisk again until well combined. Add the mashed bananas and mix until well combined.
Add the dry ingredients to the wet ingredient and stir until well incorporated. Gently fold in the walnuts, stirring with a spatula, until evenly mixed into the batter. (If your bananas are not ripe enough you may have to add an extra splash of milk if your batter is too thick.)
Transfer the batter to the greased loaf pan and transfer to the oven for 60-65 minutes until a toothpick comes out clean.
Once baked, remove banana bread from the oven and allow the loaf to cool completely in the loaf pan. Once cooled, remove from the pan and slice into 10 even slices.
This banana bread can be served immediately or stored in an airtight container for up to 5 days or in the freezer for up to 3 months.