Step 1

Preheat the oven to 350 degrees F. Lightly grease a 9X13-inch pan (see note) with cooking spray and set aside.

Step 2

For the bars, in a large bowl, cream together the butter, brown and granulated sugars, sour cream, egg, and vanilla until well-mixed, 1-2 minutes.

Step 3

Blend in the bananas and mix.

Step 4

Stir in the flour, baking soda and salt and mix for just a minute or two until combined.

Step 5

Pour the batter evenly into the prepared pan and bake for 15-20 minutes until a toothpick inserted in the center comes out clean and the top springs back to the touch.

Step 6

While the bars are baking, make the frosting by melting the butter in a saucepan or stainless steel (not nonstick) skillet over medium heat (here's a quick tutorial on browning butter). Bring the butter to a gentle simmer, turning the heat down if needed, until the solids lightly brown and the butter smells caramelly and fragrant. Immediately remove from the heat and add the powdered sugar, vanilla and milk, whisking until smooth and creamy. Add more milk if needed to achieve a thick but spreadable consistency.

Step 7

Let the bars cool for 5-10 minutes and then dollop the frosting over the warm bars and spread in an even layer.

Step 8

Cool the bars to room temperature. Cut into squares and serve. These keep well-covered for 1-2 days at room temp or refrigerated.