Cake: Preheat the oven to 375 degrees F.
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, zest, and blueberries. Scoop the batter into paper lined large muffin cups. Bake until light golden brown, 25 to 30 minutes. Cool to room temperature.
Glaze: Stir the ingredients together until smooth. Dip the tops of the muffins into the glaze.
Recipe courtesy of Gale Gand