In a large mixing bowl, beat butter with a hand mixer for about 30 seconds. Add sugar, baking soda, and salt and beat until combined. Add mashed bananas, eggs, sour cream, and vanilla and mix to combine. Add flour and mix to combine.
Pour into a greased 11-x-15-inch baking dish and spread evenly.
Bake for 25–30 minutes. Let cool in the pan.
While the bars cool, make the frosting. In a medium bowl, beat cream cheese and butter with a hand mixer until smooth. Beat in powdered sugar and vanilla and mix until combined.
Spread frosting over cooled bars and cut into squares.
STORE. Keep unfrosted Banana Bars at room temperature, but frosted ones need to be stored in the refrigerator. Keep them covered with plastic wrap or in an airtight container for up to 5 days.Make ahead of time. These can be made in advance and placed in the fridge. Frost closer to when serving for the best result. If decorated with fruit, I recommend taking it off before storing it and replacing it with fresh fruit before serving.Decorating. We like to decorate our Banana Bread Bars with banana slices. Do not put them on too early because the bananas will get brown.For a variation, top the bars with chopped walnuts, Caramel Syrup, chocolate chips, chocolate shavings, sprinkles, or berries. Make chocolate chip Banana Bars by adding chocolate chips to the batter!