01 -
Preheat oven to 375°F (190°C). Grease and flour a 15×10-inch jelly roll pan.
02 -
In a large bowl, beat together sugar, sour cream, softened butter, and eggs until creamy.
03 -
Blend in the mashed bananas and vanilla extract until well combined.
04 -
Mix in flour, baking soda, and salt. Blend for 1 minute.
05 -
Gently stir in chopped walnuts if desired.
06 -
Spread the batter evenly into the prepared pan. Bake for 20 to 25 minutes or until golden brown.
07 -
In a large saucepan, heat butter over medium heat until boiling. Allow the butter to turn a light brown color, then remove immediately from heat.
08 -
Add powdered sugar, vanilla extract, and milk to the browned butter. Whisk together until smooth; the consistency should be thicker than a glaze but thinner than traditional frosting.
09 -
Using a spatula, spread the brown butter frosting over the warm bars. The frosting will spread more easily while the bars are still warm.