Preheat your oven to 350° degrees F. Line a 15x10x1 rimmed baking sheet with parchment paper, then spray the pan and parchment liberally with cooking spray; set aside.
Meanwhile, in a medium saucepan, combine the butter and water over medium-high heat and bring to a boil, letting butter melt completely. Remove from heat and cool for 5 minutes.
In a large bowl, combine the sugars, mashed bananas, buttermilk, eggs, vanilla and banana extracts with a whisk until fully combined. At this point if you're using any mix-ins like nuts or chocolate chips, add them here.
In a medium bowl, whisk together the baking soda, salt, and flour. Gradually add the flour mixture to the wet banana mixture using a handheld electric mixer, mixing the flour in slowly until just barely combined. Slowly stream in the butter mixture until fully combined.
Pour the banana batter into the prepared pan. Carefully transfer to the oven on the center rack. Place a cookie sheet underneath the banana bar pan on a lower rack to catch any drips just in case. Bake for 18-25 minutes or until a toothpick inserted near the center comes out clean or with moist, but not wet, crumbs. Cool completely.
For the frosting: In the bowl of a stand mixer, cream together the cream cheese, butter, and vanilla extract until fluffy, about 1 minute. Gradually add in the confectioners' sugar until frosting is light and fluffy. Spread the frosting over the cooled bars in an even layer. Cut into squares and serve immediately, or cover and refrigerate. Keep bars stored covered in the fridge. Bars can keep in the fridge covered for about 4 days.