Sift flour and 1/2 cup sugar together 3 times; set aside.
Beat egg yolks at high speed of an electric mixer 4 minutes or until thick and lemon colored. Add lemon extract; beat at medium speed an additional 5 minutes or until thick. Set aside.
Beat egg whites (at room temperature) until foamy. Add cream of tartar and salt; beat until soft peaks form. Add 3/4 cup sugar, 2 tablespoons at a time; continue beating until stiff peaks form.
Sprinkle one-fourth of flour mixture over egg whites; gently fold it in. Repeat procedure with remaining flour, adding one-fourth of the mixture at a time. Divide egg white mixture in half.
Fold vanilla into half of egg white mixture. Gently fold beaten egg yolks into remaining egg white mixture.
Pour half of yellow mixture into an ungreased l0-inch tube pan; then gently add half of white mixture. Repeat procedure with remaining mixtures. Gently swirl batters with a knife to create marble effect.
Bake at 350°F for 45 to 50 minutes or until cake springs back when lightly touched. Invert pan carefully. Let cake cool in pan 40 minutes. Loosen cake from sides of pan using a narrow metal spatula; remove from pan.
Vanilla Sponge Cake: For a plain sponge cake, do not divide egg white mixture in half. Substitute vanilla extract for lemon extract.