Preheat your oven to 350°F. Lightly spray the muffin pan with the non-stick cooking spray or line with mini muffin liners. Set aside.
Stir together the vinegar and milk. Set aside.
Mash bananas and set aside.
In a separate bowl, blend the cake mix, mashed bananas, oil, and vinegar and milk mixture together until smooth. Scrape the sides of the bowl as needed.
Fold in mini semi-sweet chocolate chips.
Spoon batter into prepared muffin pan using a small ice cream scoop.
Bake on the middle rack as directed below until toothpick inserted in center comes out clean. These are quick to bake so will take about 13-15 minutes.
Remove pan from the oven and place on a wire rack. Allow muffins to cool for at least 10 minutes before removing from pan and placing directly on wire rack.