Preheat oven to 375 degrees.Spray a 10x10 casserole dish generously with cooking spray.In a large bowl, mix oats, brown sugar, baking powder, cinnamon, salt, half the chocolate chips, & half the berries. Stir well and pour into casserole dish. Add the other half of the berries & chocolate chips to the top of the mixture. Then, slice the banana & add to top.In a separate bowl, combine milk, egg, butter, & vanilla. Whisk thoroughly. Pour on top of oatmeal mixture, making sure the all oats are soaked evenly.Bake for 30 minutes. Then, remove casserole & add a tablespoon of brown sugar to the top of the casserole for added sweetness (optional.) Bake for an additional 5 or 10 minutes or until the top of the casserole is golden brown.