In a medium saucepan combine sugar, cocoa and a dash of salt. Slowly stir in hot water. Cook over medium heat, continuing to stir, until mixture comes to a boil. Boil and stir for 2 minutes.
Add milk and warm to desired temperature.
Remove mixture from the heat and stir in vanilla.
Optional: Beat with a milk frother
Top with marshmallows or whipped cream if desired.