Design Eat Repeat

0 Preheat oven to 350℉. Spray metal 9x13" pan with nonstick cooking spray and lightly dust with all-purpose flour (or cover in parchment paper). See notes below for baking in jelly roll pan.
1 In a small mixing bowl, mash bananas ([adjustable]1 ½ cups, or 2-3 large bananas[/adjustable]) with a fork until broken up. Set aside.
2 In a large bowl, cream together the softened unsalted butter ([adjustable]½ cup, i.e., 1 stick[/adjustable]), granulated sugar ([adjustable]1[/adjustable] cup), and light brown sugar ([adjustable]½[/adjustable] cup) until smooth (about 1 minute). While the mixer is on low, add in the eggs [adjustable](2 large), sour cream (¾ cup), vanilla extract (1 teaspoon), baking soda (1 teaspoon), and salt (¼ teaspoon). Add cornstarch (1 tablespoon) and all-purpose flour (2 ½ cups)[/adjustable] and mix until combined. Lastly, mix in mashed bananas ([adjustable]1 ½[/adjustable] cups) until incorporated; do not overmix.
3 Pour batter into greased 9×13 pan and bake at 350℉ for 28-30 minutes, or until toothpick comes out clean. Top should be slightly golden brown, as shown in the photo above. Allow bars to cool completely before frosting.
Frosting:
5 Note: If you're using a 9x13 pan, this recipe makes a THICK layer of cream cheese frosting. If you prefer less frosting, cut this recipe in half. If baking bars in a 13x18" jelly roll pan, I suggest making the full amount of frosting.To Make: Beat cream cheese ([adjustable]8 ounces), unsalted butter (½ cup, i.e., 1 stick), vanilla extract (1 teaspoon), and milk (1 [/adjustable]teaspoon) on medium speed until combined. Slowly add in powdered sugar (3 ½ cups) and beat smooth and creamy (about 1-2 minutes). If frosting is too thick, add more milk until desired consistency.Spread onto bars and cut into 24 squares. To help the frosting firm up, place in refrigerator. 
6 Cover and store in fridge for 3-4 days.