Step 1: Prepare Your Pan and Brown the Butter. Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan, or line it with parchment paper, leaving an overhang on the sides for easy lifting. In a light-colored saucepan, melt 1 cup (226g) of unsalted butter over medium heat. Continue cooking, stirring occasionally, until the butter turns a rich golden brown and nutty-smelling solids appear at the bottom (about 5-7 minutes). Immediately pour the brown butter into a heatproof bowl to cool slightly. This stops the cooking process and prevents it from burning.
Step 2: Combine Dry Ingredients. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, light brown sugar (packed), baking soda, ground cinnamon, and salt. Make sure there are no lumps of brown sugar. This ensures even distribution of leavening and spice throughout the banana bars.
Step 3: Mash Bananas and Mix Wet Ingredients. In a separate bowl, mash the very overripe bananas until mostly smooth (a few small lumps are fine for texture). Add the slightly cooled brown butter, large eggs, and vanilla extract to the mashed bananas. Whisk until well combined.
Step 4: Combine Wet and Dry. Pour the wet banana mixture into the dry ingredients. Mix with a spatula until just combined. Be careful not to overmix, as this can lead to tough bars. A few streaks of flour are okay.
Step 5: Bake the Bars. Pour the batter into your prepared baking pan and spread evenly. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs, but not wet batter. The edges should be lightly golden. Let the bars cool completely in the pan on a wire rack before frosting.
Step 6: Prepare the Cream Cheese Frosting. While the bars cool, in a medium bowl, using an electric mixer, beat the softened cream cheese (8 oz) and softened unsalted butter (1/2 cup) until smooth and creamy. Gradually add the powdered sugar (4 cups), one cup at a time, beating until fully incorporated and smooth after each addition. Stir in the vanilla extract (1 teaspoon). If the frosting is too thick, add milk or cream (1-2 tablespoons) one teaspoon at a time until it reaches your desired consistency.
Step 7: Frost and Serve. Once the banana bars are completely cooled, spread the cream cheese frosting evenly over the top. You can use an offset spatula for a smooth finish. Cut into 24 squares and serve. Enjoy one of the best banana recipes overripe!