Preheat the oven to 350F degrees. Spray a 9" high sided round pan or springform pan with nonstick cooking spray. Note: If using a springform pan, you will need to place a piece of foil or bake on a sheet tray to catch any of the butter/sugar mixture that may leak out the bottom.
In a medium bowl, whisk together butter and brown sugar. Spread into the bottom of the prepared pan.
Top the brown sugar mixture with sliced bananas in an even layer. Set aside.
In a large bowl, whisk together all-purpose flour, baking powder salt, set aside.
In another bowl, beat together unsalted butter, granulated sugar, egg and vanilla extract until combined.
Add in the flour mixture to the butter mixture and beat until combined. It will look crumby at this point which is normal.
Lastly, add in milk and beat until incorporated.
Spoon the batter over the bananas and gently spread using an offset spatula as the batter is thick.
Bake in the oven for about 35-40 minutes until golden or until a toothpick inserted into the center comes out clean. Please note, oven times vary so please check with a toothpick first to ensure doneness as it may take longer in your oven.
Remove from the oven and let cool in the pan for about 10 minutes (don’t skip this essential cooling down time). Run a knife along the pan's edge to ensure it is released before inverting it over a serving platter.
Let cool and dust the top with powdered sugar and serve each slice with a dollop of Cool Whip (optional).