Preheat oven to 350°F. Grease or line a loaf pan with parchment paper and set aside.
In a large bowl, mix together mashed bananas, maple syrup, nut/seed butter, vinegar, milk, and vanilla extract.
In a separate bowl, whisk flours, baking soda, cinnamon, and salt (if using).
Add flour mixture into the banana mixture and gently mix together. Add an extra tablespoon or two of milk if the batter is very dry. Mix until ingredients are almost incorporated (some dry flour pockets are OK at this point), add walnuts/chocolate chips if using, and gently mix a couple more times to evenly distribute the add-ins. At this point no flour streaks should remain. Be sure to scrape down the sides and the bottom of the bowl for dry flour.
Pour batter into the prepared loaf pan. Bake for 60 minutes, or until a toothpick inserted in the center of the loaf comes out clean. Let the bread cool for about 5 minutes in the pan, then transfer to a cooling rack. Cool completely before serving.