Mix butter and sugar together. Add eggs, sour cream, and vanilla. Mix in mashed bananas.
Add in dry ingredients and mix thoroughly.
Spread batter in a greased 9x13 baking dish and bake at 350 degrees for 25-28 minutes, or until a toothpick inserted comes out dry.
Mix frosting ingredients together and spread over cooled banana bars. Sprinkle with ½ cup chopped walnuts (optional).
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Notes
🍌How To Scale This recipe
To bake in a 9"x13" pan, make a single batch and bake for 25 minutes. Make a single batch of frosting.
To bake in a 10"x15" jelly roll pan, make 1 ½ batches and bake for 25-28 minutes. Make a single batch of frosting.
To bake in a 13"x18" half sheet pan, make a double batch — doubling all the ingredients except the eggs (use 3 eggs, as you would for 1 ½ batches) —and bake for 28-30 minutes. Make 1 ½ batches of frosting.