Preheat the oven to 325°F (163°C). Grease and line a 9×5-inch loaf pan with parchment paper or lightly flour the pan.

In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon until well combined.

In a large bowl, mash the bananas until smooth.

Add the egg, sugar, vegetable oil, buttermilk, and vanilla extract to the mashed bananas and mix until smooth and fully combined.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Avoid overmixing to keep the bread tender.

Fold in the chopped walnuts, reserving a small amount to sprinkle on top if desired.

Pour the batter into the prepared loaf pan and spread it evenly. Sprinkle the remaining walnuts over the top.

Bake for 55–65 minutes, or until a toothpick inserted into the center comes out clean.

Allow the bread to cool in the pan for about 10–15 minutes before transferring it to a wire rack to cool completely. Slice and serve.