Preheat oven to 325° F.
Butter skillet or bread pan. In a mixing bowl, whisk flour, baking soda, salt, and cinnamon until combined. Separately, whisk egg, sugar, and oil until combined. Combine wet and dry ingredients, mix well. Fold in walnuts. Add buttermilk, vanilla, and smashed bananas until well mixed. Pour into bread pan. Sprinkle walnuts on top. Cook on center oven rack for 60-75 minutes (probably closer to 75). Cool 1 hour before removing from pan and slicing.