In a small mixing bowl combine the masa harina with 1/2 cup of water mixing by hand to form a paste. In medium saucepan add the paste and the remaining 3 cups of water. Bring to boil stirring constantly. Cook slowly for 20 minutes, the mixture will thicken. While this mixture is simmering, in another saucepan add the milk and bring to a boil. Add the chocolate and cook for about 10 minutes until the chocolate has completely dissolved. Pour the chocolate into the masa harina mixture stirring until well incorporated. With a milinillo or an eggbeater, whisk until a foam forms on top, about 3 minutes. Serve in a large bowl spooning some of the foam on top.