This cocoa is not for the faint of heart. It is real chocolate, it is thick, it is rich, and it is amazing.
Once dissolved, heat the milk on medium heat just until it just starts to boil, then remove from heat. Add the chocolate immediately and begin stirring until the chocolate is completely melted.
Place the pan back on the stove on medium low/medium heat, stirring slowly, but constantly. (Do not cook the mixture over high heat because it may cause lumping.)
The mixture should thicken quickly and as soon as that happens, remove the pan from the heat so the cornstarch will not thin.