In large bowl of an electric mixer, beat butter and sugar until creamy; beat in egg, vanilla, and banana.
In another bowl, stir together flour, baking powder, and salt; gradually add to butter mixture, blending thoroughly.
Stir in walnuts and butterscotch chips.
Spread batter evenly in a greased 9-inch square baking pan.
Bake in a 350Â° oven for 35 to 40 minutes or until golden brown.
Let cool in pan on a rack, then cut into 2 1/4-inch squares.