Preheat your oven to 350°F and grease a 17.9x12.9x1-inch cookie sheet.
In a large bowl, cream the softened butter and sugar until light and fluffy, about 5-7 minutes of mixing.
Beat in the eggs, mashed bananas, sour cream, and vanilla extract until well combined and smooth.
Whisk together the all-purpose flour, baking soda, and salt in a separate bowl. Gradually add this dry mixture to the creamed mixture, stirring until blended. Avoid overmixing.
Transfer the batter to the greased baking sheet, spreading it evenly.
Bake in the oven for 20-25 minutes or until a toothpick inserted in the center comes clean. The bars should have a golden brown color and a slightly springy texture.
Once baked, remove the pan from the oven and place it on a wire rack to cool completely. Ensure that the bars are completely cooled before frosting.
While the bars are cooling, prepare the cream cheese frosting. In a small bowl, beat the softened cream cheese and butter until fluffy and well combined.
Gradually add the confectioners' sugar, one cup at a time, beating well after each addition to achieve a smooth and creamy frosting.
Stir in the vanilla extract until the frosting is thoroughly mixed.
Once the bars are cooled, frost them with the cream cheese frosting, spreading it evenly over the top.
For added decoration, garnish the bars with sliced bananas or a sprinkle of cinnamon, if desired.