preheat oven to 375 grease and add flour in 15 X 10 -inch jelly roll pan for the bars, in a large bowl beat together sugar,sour cream, butter, and eggs until creamy. Blend in bananas and vanilla extract. add flour, baking soda, salt, and blend for 1 minute. stir in walnuts.
spread batter evenly into pan. bake to 20 to 25 minutes or until golden brown
meanwhile, for frosting heat butter in a large saucepan over medium heat until boiling. Let the butter turn a delicate brown and remove from heat immediately.
add powder sugar, vanilla extract and milk. Whisk together until smooth [it should be thicker than the glaze but thinner than the frosting] using a spatula, spread the brown butter frosting over the warm bars [the frosting will be easier to spread while the bars are still warm