Break the chocolate into small pieces and set it aside.
Place the milk, cornstarch, and cocoa powder in a pot. Whisk well.
Heat over medium/low heat for a minute or so. Add the sugar and salt, and mix well.
Add the chocolate and melt, continually stirring with a wooden spoon. Bring to a boil and cook a minute or two.
You can use brown sugar instead of white, but the flavor will change slightly.