Preheat your oven to 350°F and line 6 holes of a standard 12-cup muffin pan with paper liners.
Sift flour, cocoa, baking powder, baking soda and salt into a medium bowl and whisk to blend evenly.
In a large bowl, whisk together mashed banana, brown sugar and egg white until smooth. Whisk in oil, milk and vanilla until well blended. Stir in chocolate chips. Add the flour mixture and fold very gently with a spatula until just combined. A few streaks of flour may still remain and they will get mixed in as you portion out the mixture (it is best to not over-mix a muffin batter).
Fill each greased muffin cup all the way with batter. Top with a few extra chocolate chips and bake for 16-18 minutes until muffin tops spring back when touched gently. Let cool in pan for 5 minutes before transferring individually to a wire rack to cool.