Preheat oven to 350°. Whisk together flour and baking powder., In a large bowl, mix brown sugar, melted butter and maple flavoring. Beat in eggs, 1 at a time, mixing well after each addition. Stir in flour mixture and walnuts. Spread into a greased 9-in. square baking pan., Bake until a toothpick inserted in center comes out clean, 27-32 minutes. Cool completely in pan on a wire rack. If desired, dust with confectioners' sugar. Cut into bars.