Preheat oven to 350 degrees F. Spray a 12 cup muffin tin with cooking spray or line with paper liners. (see notes below)In a large bowl combine oats, brown sugar, cinnamon, salt and baking powder.In a separate bowl whisk together mashed bananas, oil,eggs and vanilla until well combined. Whisk in milk.Pour milk mixture over oat mixture and stir well to combine. The batter is going to be very liquidy and appear like it's not holding together. It's not supposed to; just stir until everything is well combined. It's going to look very soupy.Use an ice cream scoop or a ladle to fill the prepared muffin cups all the way to the top. Try to insure each cup is filled pretty evenly with oats and liquid. You don't want to end up with a cup that is mostly liquid without a good amount of oats.Sprinkle the tops with chocolate chips and bake for about 20-25 minutes or until set and just lightly brown on the edges. (Baking time may be slightly longer if you use paper liners)If you didn't use liners, allow the cups to cool completely for easiest removal then reheat to serve warm. Or, if you used the liners, allow to set for about 5-10 minutes, then remove from pan.