Preheat your oven to 375°F (190°C) and grease a 15×10-inch pan.

In a large mixing bowl, cream together the granulated sugar and softened butter until light and fluffy.

Add the mashed bananas, sour cream, and vanilla extract; mix until well combined.

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

If using walnuts, fold them into the batter gently.

Pour the batter into the prepared pan and spread evenly. Bake for 20-25 minutes or until a toothpick inserted comes out clean.

While cooling, prepare the brown butter frosting by melting unsalted butter in a saucepan over medium heat until it turns golden brown and nutty.

In a mixing bowl, combine powdered sugar with the browned butter and milk; mix until smooth.

Frost the brownies once they have cooled slightly.