Preheat oven to 350°. In a large bowl, mix flour, confectioners' sugar and melted butter until crumbly; stir in nuts. Press onto bottom and 1/2 in. up sides of a greased 8-in. square baking dish. Bake 15-20 minutes or until light brown.
Meanwhile, in a small bowl, whisk sugar, flour, baking powder and salt. Beat in eggs, lemon juice and zest until blended.
Pour over hot crust. Sprinkle with nuts. Bake 10-15 minutes or until lightly browned. Cool completely on a wire rack. Cut into bars. Sprinkle with confectioners' sugar. Refrigerate leftovers.