This is super rich and thick, perfect with churros or Magdelenas. The serving size is approximate, depending on your sweet tooth. I have also posted a version made with unsweetened chocolate and sugar.
Pour the milk into a medium saucepan and add the cornstarch. Whisk to dissolve the cornstarch. Once the cornstarch is dissolved, heat the milk on medium heat just until it boils, then remove from heat.
Add the chocolate immediately and begin stirring until the chocolate is completely melted. If the milk cools off too fast, place the pan back on the stove on low heat to melt the chocolate.
Place the pan back on the stove on medium low heat, stirring slowly, but constantly. (Do not cook the mixture over high heat because it may cause lumping.) Taste the chocolate for sweetness and add more sugar if necessary. The mixture should thicken quickly. As soon as you see it thicken, remove the pan from the heat so the cornstarch will not thin. Ladle immediately into cups and serve piping hot.
Note: Be sure to use a clean spoon every time you taste the chocolate. Enzymes from your mouth can cause a thickened cornstarch mixture to thin.