Preheat the oven to 350 degrees . Using an electric mixer, combine the butter, 2 cups flour and the confectioners' sugar until the mixture is the consistency of coarse sand. Stir in 1/2 teaspoon lemon peel and the orange peel. Press the mixture into the bottom of a 9-by-13-inch baking pan. Bake until golden, about 25 minutes.
Meanwhile, using an electric mixer, combine the granulated sugar, eggs, lemon juice, orange juice, remaining 1 teaspoon lemon peel, remaining 2 tablespoons flour and the baking powder and mix for 1-1/2 minutes. Pour the filling into the crust and bake until the top is golden, about 40 minutes. Let cool completely. Cut into bars and dust with confectioners' sugar.