Cook water and cocoa or chocolate (finely cut) in the top part of the double boiler, directly over the fire until smooth (about 5 minutes). Place over boiling water in double boiler, add milk, sugar, and salt and cook until smooth. Beat with a beater just before serving in order to break the film that forms when milk is heated. Cocoa and chocolate may be made directly over the fire. In this case heat almost to boiling, stirring constantly, but do not boil. Serve with whipped cream if desired. Vanilla is a desirable addition to chocolate.