Sift together flour, ¾ cup of the sugar, cocoa, and salt. Set aside. In a large mixing bowl, beat egg whites with vanilla and cream of tarter at high speed until foamy. Add remaining sugar, 2 tbsp at a time, beating constantly until sugar is dissolved and whites are glossy and stand in soft peaks. Sift about ¼ c. of the reserve flour mixture over egg whites and gently fold just until flour disappears. Repeat, folding in remaining flour mixture, ¼ c. at a time. Pour into ungreased 10x4-inch tube pan. Gently cut through batter with a metal spatula.
Bake in preheated 400 degree F oven until top springs back when touched lightly with fingers, about 30 minutes. Invert cake in pan on funnel or bottle neck. Cool completely, about 1 ½ hours. Loosen cake from pan with narrow spatula or knife and gently shake onto serving plate. Drizzle with creamy mint glaze.
Creamy Mint Glaze
¾ c. confectioners' sugar ¼ tsp mint extract about 1 tbsp. milk
Stir all ingredients together until well blended, using enough milk to make mixture of drizzle consistency.