Preheat your oven to 375°F (190°C) and grease and flour a 15x10-inch jelly roll pan.

In a large mixing bowl, combine sugar, sour cream, softened butter, and eggs. Mix until smooth and creamy for 2-3 minutes.

Mix in the mashed bananas and vanilla extract until fully combined.

Sprinkle the all-purpose flour, baking soda, and salt over the wet mixture. Gently fold until just combined.

If using walnuts, fold them into the batter.

Transfer the batter to the prepared jelly roll pan and smooth it out evenly.

Bake for 20-25 minutes until golden brown and a toothpick comes out clean.

Melt butter in a saucepan until it turns golden brown and develops a nutty aroma. Remove from heat and cool slightly.

In a mixing bowl, whisk together powdered sugar, vanilla extract, and milk with the browned butter until smooth and creamy.

Spread the brown butter frosting over the cooled brownies and slice into squares.