Preheat the oven to 325°F for a glass pan or 350°F for a nonstick pan. Grease a 9x5-inch loaf pan and dust with flour.
Whisk together the flour, baking soda, and salt and set aside.
In a separate bowl, whisk together the egg, sugar, and vegetable oil until combined. Add in the flour mixture and mix just until combined, then mix in the buttermilk, vanilla, and mashed bananas until combined.
Fold in ½ cup chopped walnuts and pour the batter into the prepared loaf pan.
Top the batter with the remaining ⅓ cup of chopped walnuts and bake for 45 to 60 minutes, until a toothpick inserted into the center of the loaf comes out clean.
Cool for 10 minutes on a wire rack and remove from pan. Slice and serve warm or room temperature with or without butter. Tastes best after sitting overnight.