Sift together flour, salt and baking powder. Beat 3 egg whites until foamy and add powdered sugar beating until stiff. Set aside. Cream butter and sugar and add egg yolk, beating well. Add lemon juice and rind, dry ingredients and nuts. Fold in egg whites.
Pour into greased 9x13-inchpan.
Bake at 400 degrees F for 25-30 minutes. Cool and cut into square.