First, preheat the oven to 325 degrees. Spray an 8x5 loaf pan with non-stick spray and line it with parchment paper.
In a medium bowl, add flour, baking soda, salt, and cinnamon, and whisk just until the ingredients are combined.
In another medium bowl, add the egg, sugar, and oil, and whisk until combined.
Next, add the dry ingredients to the wet ingredients, and mix well. The mixture will be thick and dry. Add in the buttermilk, vanilla, and mashed bananas, and mix until combined.
Pour into a loaf pan, and sprinkle walnuts over the top.
Bake for 60-75 minutes. If you poke the loaf with a knife and the knife comes out clean, it is ready.
Slice and serve.