Preheat oven to 350°. Line a 9x13-inch baking dish with foil and spray with cooking spray. Set aside.
In a bowl combine melted butter and sugar. Using a hand mixer, mix until combined. Add eggs and vanilla and continue mixing until incorporated. Stir in salt, flour, and banana until batter is smooth. Spread evenly in prepared pan.
Bake 25–30 minutes until center is set. Cool before frosting.
For frosting
In a medium saucepan, melt butter. Add brown sugar and milk and stir until mixture comes to just a boil. Remove from heat, allow to cool 5 minutes.
Whisk in confectioners’ sugar.
For assembling
Spread on top of blondies quickly—frosting begins to set fast. Allow frosting to set completely before cutting into squares.