Preheat your oven to 350°F (175°C) to ensure even baking.
Grease a 9x13 inch baking pan with butter or non-stick spray, or line the bottom with parchment paper for easy removal.
In a large mixing bowl, cream together 1 cup softened butter and 1 cup sugar until light and fluffy.
Add 2 eggs, 2 teaspoons vanilla extract, and 3-4 mashed ripe bananas; mix until well combined.
Stir in 1 cup sour cream to keep the bars moist and add tanginess.
In a separate bowl, mix 2 cups all-purpose flour, 1 teaspoon baking soda, and 1/2 teaspoon salt.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Fold in 1 cup chopped walnuts if using.
Pour the batter into the prepared pan and spread evenly.
Bake for 25-30 minutes until a toothpick inserted in the center comes out clean and edges are golden brown.
Remove from oven and cool on a wire rack completely before frosting.
Prepare frosting by mixing 4 cups powdered sugar, 1 teaspoon vanilla extract, and 3 tablespoons milk until smooth.
Spread frosting generously over cooled bars.