PreHeat oven to 375F.  Prepare a 15×10-inch sheet cake pan with cooking spray and line with parchment paper. Set aside

For the bars:

In a large mixing bowl, beat together sugar, sour cream, butter, and eggs until creamy with an electric mixer( or use your stand mixer with a paddle attachment). Add the mashed bananas and vanilla extract and blend.

Then add dry ingredients, flour, baking soda, salt, to the wet ingredients, and blend for another 1 minute.

Stir in the walnuts (optional).

Spread the cake batter evenly into the prepared pan.

Bake for 20-25 minutes or until golden brown in a preheated oven.

While the bars are baking make the browned butter frosting. Once the bars are cooled spread with the browned butter frosting.

For the browned butter frosting

Heat the butter in a large saucepan over medium heat until boiling.  Let the butter turn light brown and remove from heat immediately. Make sure to watch the butter while cooking it can quickly go from browned to burnt.

Add powdered sugar, vanilla and milk. Mix until smooth. Immediately spread the brown butter frosting over the cake while it is still warm, it spreads much easier.