Make the batter: beat together the sugar, sour cream, softened butter, and eggs until creamy. Preheat the oven to 375 F.
Mix in the mashed bananas and vanilla extract.
Whisk in the flour and baking soda until smooth. Mix in the walnuts.
Spoon the batter into a greased 9x13-inch baking dish.
Bake for 25 mins or until slightly golden brown
Make the glaze: melt the butter in a medium-sized saucepan over low heat and cook until golden brown. Turn off the heat and mix in the powdered sugar, vanilla, and milk.
Spread the glaze over the brownies while still warm. Let them cool completely and cut into squares.