Preheat oven to 350. Prepare 8x8 or 9x9 baking dish with cooking spray.
In a bowl combine melted butter and brown sugar. When combined add in egg and vanilla. Continue mixing until incorporated, then add salt, flour and banana.
Spread in dish, bake 25-30 minutes (until fork comes out of the middle clean.)
Let blondies cool while you make the easy brown sugar frosting.
Brown Sugar Frosting:
Put butter into a microwave safe dish and heat/melt butter for 1 minute. It should be bubbling.
Add milk and brown sugar. Stir and let sit for 5 minutes.
Whisk powdered sugar in.
Spread over blondies and allow to cool before cutting into squares.