• Arrange oven rack in the middle of the oven and preheat the oven to 350ºF. Coat a 9x13-inch baking pan with cooking spray. Line the bottom with parchment paper so it extends over the two long sides of the pan.
• Place the bananas in a shallow bowl, and mash with a fork or potato masher until mostly smooth (about 1 cup).
• Beat the butter and sugar in a large bowl using an electric mixer on medium speed until pale and fluffy, 2 to 3 minutes. Beat in the eggs, one at a time until well blended. Beat in the sour cream, vanilla extract, and salt until blended, stopping to scrape down sides of the bowl as necessary.
• Reduce the mixer speed to slow and beat in the baking soda, and then then flour until almost combined. Add the bananas and beat on the low until just combined. Scrape the batter into the prepared pan and spread evenly.
• Bake until light golden brown and a tester inserted into the center comes out clean, 30 to 35 minutes. Transfer the pan on a wire rack and let the bars cool completely.

Meanwhile, make the frosting.

• In a medium bowl, beat the cream cheese and butter on medium speed until light and fluffy, about 3 minutes and stopping to scrape down the sides as necessary. Beat in the vanilla and salt until blended.
• Reduce the mixer speed to low, and slowly beat in the powdered sugar until blended. Increase the mixer speed to medium-high and beat until smooth and fluffy, about 3 minutes.
• To ice the bars: Grab the excess parchment and transfer the banana bars from the pan to a cutting board; remove and discard the parchment.
• Spoon the frosting onto the bars and spread with an offset spatula into an even layer. Cut into 20 pieces and serve.