To prepare cake, lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and 3/4 cup sugar, stirring with a whisk.
Place egg whites in a large bowl; beat with a mixer at high speed until foamy. Add cream of tartar and salt; beat until soft peaks form. Add 3/4 cup sugar, 2 tablespoons at a time, beating until stiff peaks form. Beat in vanilla and juice.
Sift 1/4 cup flour mixture over egg white mixture; fold in. Repeat procedure with remaining flour, 1/4 cup at a time. Gently spoon half of cake batter into a large bowl. Sift cocoa over half of batter; gently fold in.
Spoon half of plain batter into an ungreased 10-inch tube pan, spreading evenly. Top with half of chocolate batter; repeat layers. Swirl batters together with a knife. Bake at 325° for 50 minutes or until cake springs back when lightly touched. Invert pan; cool completely. Loosen cake from sides of pan using a narrow metal spatula. Invert onto plate.
To prepare glaze, place chocolate and hot water in a small glass bowl. Microwave at high 1 minute or until almost melted; stir until smooth. Drizzle glaze over cake. Chill cake 10 minutes.