• 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.
• In a small mixing bowl, mash bananas (1 ½ cups, or 2-3 large bananas) with a fork until broken up. Set aside.
• In a large bowl, cream together the softened unsalted butter (½ cup, i.e., 1 stick), granulated sugar (1 cup), and light brown sugar (½ cup) until smooth (about 1 minute). While the mixer is on low, add in the eggs (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) and mix until combined. Lastly, mix in mashed bananas (1 ½ cups) until incorporated; do not overmix.
• 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:

• 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 (8 ounces), unsalted butter (½ cup, i.e., 1 stick), vanilla extract (1 teaspoon), and milk (1 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.
• Cover and store in fridge for 3-4 days.