Combine 1/2 cup sugar with the cocoa and flour and sift 3 times.
In a clean dry bowl, whip the egg whites and salt until foamy. Add the cream of tartar and beat till soft peaks form. Add the 1/4 cup of the sugar and beat until stiff but not dry. They will be slightly glossy. Add the vanilla and fold in.
Gradually fold in the dry ingredients. Spoon batter into an ungreased 10-inch tube pan and bake for 35 minutes. Cool upside down over a bottle. After the cake has cooled use a dull knife and go around the walls of the pan to loosen the cake. Turn it out and brush off any loose crumbs. Make the icing by whisking all the icing ingredients together. Drizzle cake with the icing and let set 1 hour before serving.