Preheat oven to 350°. Grease a 9" round baking pan very well, set aside.
Combine melted butter and brown sugar. Press mixture into bottom of the pan. Arrange banana rounds on top of brown sugar mixture. (This will ultimately be the top of your cake.)
In a large bowl, combine all dry ingredients and whisk very well for one minute. Add in the wet ingredients and stir until completely combined and no lumps remain. Don't over mix. Pour cake mixture into prepared pan on top of bananas. It will fill almost to the top of the pan. RECIPE EDITED TO ADD: Based on reader's reviews, it seems some have had the batter spill over while baking, and some have had great success. To prevent this, simply place the cake pan on a baking sheet when it goes into the oven to catch anything that spills over.
Bake for 42-47 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Let cake cool for 5 minutes in pan then turn out on a large plate or platter. (If you wait too long, the caramelized brown sugar will stick to the pan!)
Cake is best served warm out of the oven but will stay moist and fresh for up to 5 days in an airtight container. This cake is able to be made the night before the day needed for consumption.