Heat oven to 350 degrees F and coat a 12-count non-stick muffin tin with cooking spray.
In a large bowl, mash the bananas with a fork. Add the oats and milk. Stir until evenly combined. Let soak about 5 to 10 minutes, or until most of the liquid is absorbed.
To the oat mixture, add the maple syrup, egg, canola oil, baking powder, cinnamon, vanilla and salt. Stir until thoroughly combined. Stir in the walnuts and chocolate or chocolate chips.
Fill each muffin cup with the batter, evenly distributing between the 12 muffin cups. Bake for 30 to 35 minutes or until lightly browned and tops spring back when lightly touched in the middle on top. Cool in pan for 10 minutes, then remove to cooling rack.
Refrigerate leftover muffins for up to 2 days or freeze for up to 3 months in an airtight, resealable bag.