In a medium bowl, mix confectioners' sugar, cake flour, and cocoa. Sift together 2 times, and set aside.
In 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 while continuing to mix. Fold in flour mixture, a little at a time, until fully incorporated. Stir in vanilla. Pour into a 10 inch tube pan.
Bake at 325 degrees F (165 degrees C) for approximately 1 hour, or until cake springs back when touched.