Preheat oven to 325 degrees, spray an 8x5 loaf pan with nonstick spray and line it with parchment paper if desired.
In a medium bowl add flour, baking soda, salt, and cinnamon and whisk just until combined.
In another medium bowl add egg, sugar, and oil and whisk until combined.
Add the dry ingredients to the wet ingredients and mix well, the mixture will be thick and dry.
Add in the buttermilk, vanilla, and mashed bananas and mix until combined.
Pour into a loaf pan, and sprinkle walnuts over the top.
Bake on the center oven rack for 60-75 minutes or until the knife comes out clean.
Video
Notes
You can make your own buttermilk by adding 1/2 tablespoon white vinegar or lemon juice to 1/2 cup milk and let sit for 5 minutes. (The recipe only uses 2 tablespoons)
To freeze let the banana bread cool completely then wrap the loaf in plastic wrap followed by aluminum foil or a large ziplock freezer bag. Freeze for up to 3 months.