Preheat oven to 350°. In a large bowl, beat butter, brown sugar and peanut butter until blended. Gradually beat in egg, banana and vanilla. In another bowl, whisk flour, baking powder and salt; gradually add to butter mixture, mixing well.
Spread into an 8-in. square baking dish coated with cooking spray. Bake until a toothpick inserted in center comes out clean, 25-30 minutes. Cool on a wire rack. Cut into bars. If desired, dust with confectioners' sugar.