Place egg whites in a large bowl; let stand at room temperature for 30 minutes. Sift together confectioners' sugar, flour and cocoa three times; set aside.

Add cream of tartar and salt to egg whites; beat on medium speed until soft peaks form. Gradually add sugar, about 2 tablespoons at a time, beating on high until stiff glossy peaks form and sugar is dissolved. Gradually fold in flour mixture, about 1/2 cup at a time.

Spoon into an ungreased 10-in. tube pan. Cut through batter with a knife to remove air pockets. Bake on the lowest oven rack at 375° for 35-40 minutes or until lightly browned and entire top appears dry. Immediately invert pan; cool completely, about 1 hour.

Run a knife around side and center tube of pan. Remove cake to a serving plate.

In a large bowl, combine the first five frosting ingredients; cover and chill for 1 hour. Beat until stiff peaks form.