Combine first 3 ingredients and sift 6 times; set aside.
Combine egg yolks and sugar in a large mixing bowl; beat at high speed of an electric mixer until mixture is thick and lemon colored. Reduce speed to medium and gradually add in vegetable oil. Add flour mixture and mix until well blended. Stir in lemon rind and juice.
Beat egg whites (at room temperature)and salt until stiff peaks form. Fold whites into batter. Pour batter into 4 wax paper-lined and greased 8-inch round cakepans.
Bake at 325 for 20-25 minutes or until cake springs back when lightly touched. Cool in pans 10 minutes; loosen cake from sides of pan and remove from pans. Pull off wax paper and cool layers completely on wire racks.
Spread Orange-Lemon Frosting between layers and on top and sides of cake. Store in refrigerator until serving time. Garnish with orange slices and fresh mint leaves if desired.
Cream butter; add egg yolks and beat well. Add powdered sugar and grated rind alternately with juices, beating well.