In a mixer fitted with a whisk attachment (or using a hand mixer), whip the egg whites until foamy. Add the cream of tartar and salt and continue whipping until soft peaks form. With the mixer running, gradually add 1 cup of the sugar and continue whipping until stiff and the sugar has dissolved, about 30 seconds.
Sift the remaining 1/2 cup sugar with the pre-sifted cake flour 3 times, to aerate the mixture. Fold into the egg whites, then fold in the vanilla, and zest.
Spoon the batter into ungreased mini tube pans. Bake until light golden brown, 25 to 30 minutes. Cool by turning the cake (in the pan) upside down until it cools to room temperature. Run a long, sharp knife blade around the cakes to loosen, then knock the cakes out onto a plate, or work them out with your fingers. The outside of the cake will remain in the pan leaving the cake whiter and more angelic looking.
Make the glaze: Stir the ingredients together until smooth. Dip the tops of the cakes in the glaze. Let set for at least 30 minutes before serving, or until the icing is hard. Serve with sorbet.