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 Green Creme de Menthe to simmer for about 10 minutes, stirring often, until it is thick, glossy and completely smooth. Serve sTeaming hot in Coffee mug. Serves six.