Preheat the oven to 350 degrees F. Spray a 9x13 inch pan with nonstick spray, or line with parchment paper. I like to line with paper so that I can lift the bars out later for easy slicing.

In a medium saucepan, melt 1 cup of butter over medium heat. Once melted, the butter will begin the bubble and foam. The bubbles will eventually subside, and the butter will start to turn brown. This is easiest to see in a stainless steel or ceramic pan. Turn off the heat as soon as the butter turns a light golden brown.

When the butter is still bubbly and hot, add in the ground cinnamon. Let it toast in the pan for just a few seconds, then remove the pan from heat.

Stir in 1 cup sugar and 1 cup packed brown sugar.

Add in your mashed banana, vanilla and kosher salt.

Add 1 egg and stir vigorously right away so that it doesn't curdle. If your butter/sugar mixture is still really hot, consider waiting a couple minutes to add the egg.

Add flour, spooned and leveled, but don't stir yet. Add 1 teaspoon baking powder to the flour and use the teaspoon to stir it into the flour.

Stir in the flour until most of the lumps are gone, but do not over beat. Just like brownies, you want to get all the ingredients incorporated, but you don't want to mix it so much that you end up with tough bars. Add 1 ½ cups of chopped walnuts or other mix-ins, leaving a half cup to sprinkle over the top of your bars.

Pour the batter into the prepared pan and spread to the edges.

Sprinkle ½ cup walnuts over the top.

Bake at 350 for about 25-30 minutes. They are done when a toothpick inserted in the center comes out with no wet batter on it.

Let cool before lifting the paper from the pan and slicing.