1. Heat oven to 375 degree F. Sift flour, cocoa and 3/4 cup granulated sugar together 3 times; set aside.
2. Beat egg whites at low speed in a large mixer bowl until foamy, about 5 minutes. Add vanilla, cream of tartar and salt. Gradually increase speed to medium while beating in remaining 3/4 cup granulated sugar, 1 tablespoon at a time, 5 minutes. When sugar is incorporated, continue beating to stiff peaks, 2 minutes more.
3. Sift one third of dry ingredients over whites; gently fold in with rubber spatula. Repeat process 2 more times.
4. Pour batter into ungreased 10-inch tube pan. Cut through batter to remove air pockets. Bake 40 to 45 minutes, until top springs back when gently pressed. Invert and hang pan on neck of bottle to cool.
5. To unmold, run a thin knife around side of pan and tube; invert pan. Turn cake upright onto serving plate. Garnish with confectioners' sugar, if desired. Makes 14 servings.