Preheat oven to 375 degrees F and grease a 15x10-inch jelly roll pan, or a 9x13’’ pan (depending on if you want thin “sheet cake” style bars or thicker bars).
For the bars:
In a large bowl, cream together the sugar and butter. Beat in the sour cream and eggs, bananas and vanilla extract.
Add the flour, baking soda, salt, and mix just until moistened. Stir in walnuts, if using.
Spread batter evenly into greased pan. For a jelly roll pan, bake for 18 to 23 minutes or until a toothpick inserted in the center comes out clean or with few crumbs. For a 9x13’’ pan, bake for 23-28 minutes or until a toothpick inserted in the center comes out clean or with few crumbs.
For the Glaze:
(I usually cut the glaze recipe in HALF if making these in a 9x13’’ pan).
In a large skillet heat the butter over medium heat. Allow it to come to a summer or very low boil and turns a delicate brown (watch carefully so you don't burn the butter) and remove from heat immediately.
Whisk in the powdered sugar, vanilla extract and milk until smooth. Pour the glaze over the warm bars (this is easiest when the bars are warm). Spread gently with the back of a spoon.