Banana Bread Brownies  1-1/2 c. sugar  1 c. sour cream  1/2 c. butter, softene  2 eggs  1-3/4 (3 or 4) ripe bananas, mashed  2 tsp. vanilla extract  2 c. all purpose flour  1 tsp. baking soda  3/4 tsp. salt  1/2 c. chopped walnuts (optional)  Brown Butter Frosting:  1/2 c. butter  4 c. powdered sugar  1-1/2 tsp. vanilla extract  3 tbsp. milk