Preheat oven to 375 degrees and line a muffin tin with 12 muffin papers or cupcake liners.
Mash the bananas, then use a whisk or electric hand mixer (on low) to combine the bananas with the eggs, apple cider vinegar, maple syrup, coconut oil, nut butter, and vanilla (option: you can also leave bananas whole and add all the ingredients to blender and puree).
In a separate bowl combine the flours, cinnamon, salt and baking soda.
Mix the dry ingredients with the wet ingredients on low speed or by hand until "just combined" (batter will be thick due to the coconut flour). Add walnuts if desired.
Transfer half the batter into the prepared papers.
Bake the bread in the preheated oven for 16-18 minutes.