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).
Video
Notes
Sheet Pan Banana BarsTo make a 13x18" half sheet pan of banana bars, double all batter ingredients except for eggs (use 3 eggs). Spread in a greased half sheet pan and bake for 28-30 minutes.You don't need to double the frosting recipe, as that would be too much. You can either use the same amount and spread it a little thinner or make 1.5 batches of frosting.Cover banana bars with plastic to keep them fresh for up to 5 days.