Stir together sugar, cocoa and salt in a medium saucepan, stir in water.
Cook over medium heat, stirring constantly, until mixture comes to a boil.
Boil and still 2 minutes.
Add milk; stirring constantly, heat to serving temperature.
Do Not Boil.
Remove from heat; add vanilla.
Beat with rotary beater or whist until foamy.
Serve topped with marshmallows or whipped cream, if desired.
Five 8-oz. servings.