Preheat oven to 350°F (175°C).

Grease a 9x9 inch baking pan.

In a bowl, mix sugar, sour cream, and butter until smooth.

Add eggs, bananas, and vanilla; mix well.

In another bowl, combine flour, baking soda, and salt.

Fold dry ingredients into wet mixture until just combined.

Add walnuts if using.

Pour batter into prepared pan and bake for 25-30 minutes.

Let cool, then prepare frosting by creaming butter, adding powdered sugar, vanilla, and milk until spreadable.

Frost cooled brownies and slice.