Preheat oven to 350 degrees F (175 degrees C). Grease and flour your loaf pan.
In a large bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.
In a separate bowl, whisk together the vegan buttermilk, avocado oil, brown sugar, mashed banana, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until just combined. Do not overmix.
Fold in the walnuts, if using.
Pour the batter into the prepared loaf pan and bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for 10 minutes before removing to a wire rack to cool completely.