Preheat the oven to 375°F (190°C) and grease a 15x10-inch baking pan.

Combine sugar, sour cream, softened unsalted butter, and eggs in a mixing bowl. Beat until creamy.

Fold in mashed bananas and vanilla extract until incorporated.

In a separate bowl, whisk together flour, baking soda, and salt. Gradually add to the wet ingredients and mix until just combined.

Fold in walnuts or chocolate chips if using.

Pour the batter into the prepared pan and spread evenly.

Bake for 20-25 minutes until the top is golden and a toothpick comes out with a few moist crumbs.

For the frosting, melt butter in a saucepan over medium heat until golden brown and nutty.

Whisk in powdered sugar, vanilla extract, and milk until smooth.

Spread the warm frosting over brownies and let cool completely before cutting into squares.