In a large mixing bowl, mix together 2 sticks of butter, brown sugar, eggs, 1 teaspoon of vanilla, flour and baking powder. Grease and flour baking pan and pour batter in. Bake for 25 minutes in a preheated 350 degree oven.
For banana topping heat the sugar, remaining 4 teaspoons butter, bananas and remaining 1 teaspoon vanilla for 5 minutes. Spread over blondies.