Preheat oven to 350° F. Line an 8x8 baking dish with parchment paper or foil and spray with cooking spray. Set aside
In a large bowl combine the melted butter and brown sugar. Mix until combined. Add vanilla and egg, mix until fully incorporated. Add flour and stir until combined. Next add the mashed banana and chocolate chips. Stir just until combined without over mixing. Pour into greased pan. Bake for 22-28 minutes.
Cool for at least 10 minutes before cutting. Cut into 9 large bars or 16 small squares. Store covered in fridge for up to 1 week.