In large saucepan, combine sugar, cocoa and salt. Add hot water slowly, mixing well. Cook over medium heat, stirring constantly, until mixture boils. Boil and stir 2 minutes. Add milk; heat to serving temperature, stirring occasionally. DO NOT boil. Remove from heat; add vanilla, cinnamon if desired, and whip with a whisk. Serve hot, with whipped cream or marshmallows if desired. This recipe can be doubled and kept in crockpot for easier serving to large groups.