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

In a large mixing bowl, combine sugar, sour cream, softened unsalted butter, and eggs. Cream these together using an electric mixer on medium speed for 2-3 minutes until smooth.

Mash the ripe bananas until smooth, then fold them into the creamed mixture with pure vanilla extract. Mix gently on low speed to combine.

In a separate bowl, sift together the all-purpose flour, baking soda, and salt. Fold this dry mixture into the banana batter using a spatula.

Pour the batter into the prepared jelly roll pan and spread it evenly.

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

In a saucepan, melt butter over medium heat to brown it, stirring frequently for 5-7 minutes until golden and nutty.

Remove browned butter from heat and whisk in powdered sugar, vanilla extract, and enough milk to achieve desired consistency.

After brownies cool for 10-15 minutes, spread the brown butter frosting evenly over the top.