• Heat oven to 375 degrees.  Grease and flour a 10 x 15 baking sheet.  Line with parchment paper if desired.
• Mash the bananas with a fork.
• In a large mixing bowl, beat sugar, sour cream, and butter until creamy.
• Mix in two eggs until combined.
• Blend in the mashed bananas and vanilla and mix for about one minute or until combined.
• Add flour, baking soda and salt and blend until combined.
• Fold in chopped walnuts and pour into prepared baking sheet.
• Bake 20-25 minutes, or until golden brown.  Brown butter and start frosting while the bars are baking. Remove from oven and let cool slightly until just warm.

Browned Butter Frosting:

• Cook butter in saucepan over medium high heat until it reaches a low boil. Lower heat and continue to simmer until golden brown and fragrant.  Allow it to cool slightly before mixing.
• Stir in the powdered sugar, vanilla and milk.  Add more or less milk to make a spreadable frosting.
• Pour frosting over warm banana bars and spread with a spatula.  Let cool completely, slice, and enjoy!!
• NOTES: *Banana bars can be made ahead of time and actually taste even better the next day as the flavors continue to develop. *Banana bars can be frozen by wrapping individual bars tightly in plastic wrap.  Freeze for up to three months. *Browning the butter is absolutely worth the extra time as it gives a richer, nutty flavor to the bars. *Add 1 cup chocolate chips if desired for chocolate lovers!