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, peanut butter, vinegar, milk, and vanilla extract.
Sift flour, cocoa powder, baking soda, and baking powder on top of the banana mixture. Use a rubber spatula to gently fold together. Add an extra tablespoon or two of milk if the batter is very dry.
Mix until ingredients are just incorporated and no flour streaks remain. Be sure to scrape down the sides and the bottom of the bowl for dry flour. Add chocolate chips and gently fold a few times to disperse.
Pour batter into the prepared loaf pan. Bake for one hour or until a toothpick inserted in the center of the loaf comes out clean (can take up to 90 min depending on the size of your pan). Let the bread cool for about 10 minutes in the pan, then transfer to a cooling rack. Cool completely before serving.
Notes
Maple Syrup: You can decrease the maple syrup to 1/4 cup for a subtly sweet banana bread or if you are adding lots of chocolate chips.