Prepare mix ahead of time. One serving is 1/4 cup mix per cup of milk.
Bring milk to a simmer in a saucepan over medium heat. Whisk cocoa/sugar mix into the milk. Add the chopped chocolate and reduce heat to low and simmer, stirring, until the chocolate melts and the cocoa is thick, about 2 minutes. Divide among mugs and garnish each with homemade marshmallows or whipped cream.