Preheat your oven to 375°F (190°C) and prepare a 15x10-inch jelly roll pan by greasing it and dusting with flour.

Cream together 1 cup of sugar, ½ cup of sour cream, and ½ cup of softened butter using an electric mixer until light and fluffy. Add in 2 large eggs and mix until well combined.

Mash 2 medium ripe bananas in a separate bowl and mix into the creamy egg mixture along with 1 teaspoon of vanilla extract.

In another bowl, sift together 1 cup of all-purpose flour, 1 teaspoon of baking soda, and ½ teaspoon of salt. Slowly add this to the wet ingredients, stirring until just combined.

Gently fold in ½ cup of chopped walnuts into the batter. Pour the batter into the prepared pan and spread it evenly.

Bake for 20-25 minutes until the top turns golden brown. Check for doneness using a toothpick.

Allow brownies to cool in the pan for about 10 minutes before frosting.

For the frosting, melt ½ cup of butter in a saucepan over medium heat, stirring until golden brown. Remove from heat, then whisk together with 2 cups of powdered sugar, 1 teaspoon of vanilla extract, and 2 tablespoons of milk until smooth.

Pour the frosting over the cooled brownies and spread it evenly. Allow the frosting to set before slicing.