In a very large mixing bowl allow egg whites to stand at room temperature for 30 minutes. Meanwhile, sift together flour and 3/4 cup sugar 3 times; set aside.
Add vanilla, cream of tartar, and salt to egg whites. Beat with electric mixer on medium to high speed until soft peaks form (tips curl). Gradually add 3/4 cup sugar, 2 tablespoons at a time, beating until stiff peaks form (tips stand straight). Sift one-fourth of the flour mixture over white mixture; fold in gently. If bowl is too full, transfer to a larger bowl. Repeat with remaining flour mixture, using one-fourth of the flour mixture each time. Transfer half of the batter to another bowl.
In a small mixing bowl beat egg yolks on high speed for 6 minutes or until thick and lemon-colored. Fold in 1-1/2 teaspoons lemon peel. Gently fold yolk mixture into half of the egg white mixture. Alternately spoon yellow batter and remaining white batter into an ungreased 10-inch tube pan. Swirl metal spatula through batters to marble.
Bake on the lowest rack in a 350 degree F oven for 40 to 45 minutes or until top springs back when lightly touched. Immediately invert cake in pan; cool completely. Loosen sides of cake from pan; remove. Place cake upside down on a plate; frost with Tangy Lemon Frosting. Sprinkle top with finely shredded lemon peel, if desired. Cover and chill up to 24 hours. Makes 12 servings.
Tangy Lemon Frosting
Beat 1/2 cup margarine or butter with an electric mixer until softened. Beat in 5-1/2 cups sifted powdered sugar, 1/2 teaspoon finely shredded lemon peel, and 1/3 cup lemon juice, adding additional lemon juice, if necessary, to make of spreading consistency.