Preheat oven to 375 degrees F. For the white batter, sift the flour and icing sugar into a bowl, then set aside. In another bowl, beat the egg whites till frothy; add cream of tartar, vanilla, and salt. Beat until soft peaks form. Gradually beat in granulated sugar until stiff peaks form. Gently fold in flour mixture in three stages.
For the yellow batter, sift flour and baking powder into a bowl, then set a side. In another bowl, beat the egg yolks, lemon juice and water until almost as thick as shaving cream, about 5-6 minutes. Gradually beat in granulated sugar. Gently fold in flour mixture in three stages.
Alternately spoon the yellow and white batters into an ungreased 9- or 10-inch, angel food cake tube pan.
Bake 35-40 minutes. Invert on a cake rack and cool. Carefully unmould and set on a cake plate. Dust with icing sugar and serve slices with ice cream or whipped cream, if desired.