Ingredients:   1 1/2 cups egg white 1 cup sifted cak flour or sifted all-purpose flour 1/4 cup granulated sugar 2 teaspoon vanilla 1 1/2 teaspoon cream of tartar 1/4 teaspoon salt 1/4 cup granulated sugar 6 egg yolks 1 1/2 teaspoons finly shreded lemon peel Tangy Lemon Frosting   Directions:   in a very large mixing bowl allow egg whites to stand at room temprture for 30 minutes.  Manwhile, sift together flour and 1/4 cup sugar 3 times.  Set aside.  Add vanilla, cream of tartar, and salt to egg whites. Beat with mixer on medium to high sped till soft peaks form.  Gradually add 1/4 cup sugar, 2 tablespoons at a time, beating till stiff peaks form.    Sift one-fourth of the flour mixture over egg white mixture; fold in gently.  Repeat with remaining flour mixture, using on-fourth of the mixture each time.  Transfer half of the batter to another bowl.   In a small bowl beat egg yolks on high speed about 6 minuts or till thick and lemon colored.  Fold in 1 1/2 teaspoon lemon peel.  Gentlyfold into half of th white batter.  Alternately spoon yellow batter and remainng white batter into an ungreased 10-inch tub pan.  Swirl a metal spatula through batters to marble.   Bake on th lowest rack at 350 degrees for 40 to 45 minutes or till top springs back when lightly touched.  Immediately invrt cake in pan; cool compleely.  Remove from pan.  Place upside down on a plate and frost with Tangy lemon frosting.  Sprinkle with lemon peel shreds.   Tangy Lemon Frosting:  In a large mixing bowl beat 1/2 cup butter till softened.  Beat in 5 1/2 cups sifted podered sugar, 1/2 teaspoon finely shreded lemon peel, and 1/3 cup lemon juice.  Beat in additional lemon juice, if mecessary to make it spreading consistency.   nutritional information pr srving:  424 caloris, 5 g protien, 79 g carbohydrates, 10 g fat (5 g saturated), 127 mg cholesterol, 189  mg sodium