Shift the cocoa and sugar together into a medium-sized saucepan. Dissolve the cornstarch in the water, and stir into the cocoa and sugar until it is a smooth paste. Begin heating the mixture, stirring it with a whisk, and gradually pour in the milk. Continue stirring with the whisk as you bring the liquid to a simmer. Allow the chocolate to simmer for about 10 minutes, stirring often, until it is thick, glossy and completely smooth. Serve steaming hot in coffee mug. Serves six.