a medium bowl, mix confectioners' sugar, cake flour, and cocoa.
Sift together 5 times, and set aside.
a clean large bowl, whip egg whites, salt, and cream of tartar
with an electric mixer on high speed until very stiff. Fold in
white sugar 2 tablespoons at a time. Fold in flour mixture, a
little at a time, until fully incorporated. Stir in vanilla. Pour
into a 10 inch tube pan.