Preheat your oven to 325 degrees Fahrenheit. Spray an 8x5 loaf pan with nonstick spray and line it with parchment paper if desired.
In a medium bowl, add the all-purpose flour, baking soda, salt, and cinnamon. Whisk these together just until combined.
In another medium bowl, combine the egg, sugar, and vegetable oil. Whisk them together until well blended.
Add the dry ingredients from your first bowl into the wet mixture. Stir well until everything is mixed together.
Fold in the buttermilk, vanilla extract, and mashed bananas into your batter. Mix just until combined.
Pour your batter into the prepared loaf pan and sprinkle chopped walnuts over the top generously.
Place your loaf pan on the center rack of your preheated oven and bake for about 60-75 minutes or until a knife inserted into the center comes out clean.