Preheat the oven to 375 degrees F. Grease a jelly roll pan and set aside
With an electric mixer, cream the butter and sugar together until well blended.
Add the eggs and blend in.
Mix in the sour cream, mashed bananas, and vanilla until the mixture is well combined.
Sift together the dry ingredients and add to the banana mixture and mix until blended.
Spread in the prepared jelly roll pan.
Bake in the preheated oven for 20 minutes.
Remove and cool completely
While the bars are cooling make the frosting.
Combine the ingredients in a medium bowl, beat with an electric mixer until well smooth and creamy. Spread the frosting over the top of the bars.