In a small mixing bowl, cream butter and confectioners' sugar, gradually beat in flour. Press onto the bottom of an ungreased 8-in. x 4-in. 2-in. loaf pan. Bake at 325º for 14-16 minutes or until set and the edges are lightly browned.
For filling, in a mixing bowl, beat the egg, sugar, lemon juice, flour and baking powder until frothy. Pour over warm crust. Bake for 18-22 minutes or until lightly browned. Cool on a wire rack. Dust with confectioners' sugar. Cut into bars.