on racks for 10 minutes. Remove from pans; cool completely on racks. To assemble cake, place one cake layer on serving plate. Spread with Cocoa Icing. Top with second cake layer and spread a layer of frosting. Top with third cake layer and frost sides and top with remaining frosting. COCOA ICING: Combine butter, and chocolate in the top of a double boiler. Place over hot water; stir until melted. Remove from heat; cool to room temperature. Sift confectioners' sugar into a large mixing bowl. Make a well in the center and add egg white, vanila, and lemon juice. Pour in the chocolate mixture. Blend until smooth, using an electric mixer set at medium speed. Add milk to make frosting of a spreading consistency.