Preheat oven to 325°F (165°C).
Grease a 9×5-inch loaf pan and line with parchment paper if desired for easy removal.
In a medium bowl, whisk together flour, baking soda, salt, and cinnamon.
In a large bowl, mix mashed bananas, sugar, oil, eggs, and vanilla extract until smooth and well combined.
Add the dry ingredients to the wet ingredients and stir until just combined.
Gently fold in the buttermilk until incorporated, being careful not to overmix.
Fold in the chopped walnuts.
Pour the batter into the prepared loaf pan and sprinkle extra walnuts on top if desired.
Bake for 60 to 70 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.