In a small saucepan, combine the sugar and cocoa. Stir in the water. Bring to a boil over moderate heat. Lower the heat and simmer for 2 minutes, whisking constantly.
Stir in the milk. Increase the heat to moderate; cook until heated through. Do not boil. Pour into a cup. Top with several marshmallows and add the cinnamon stick as a stirrer if you like. Makes 1 cup.