In a medium saucepan whisk together the cocoa powder, sugar, salt, and water. Stirring constantly, bring this chocolate mixture to a boil. Simmer for 1- 2 minutes, monitor carefully, to prevent burning.
Stir in the milk, maple syrup and vanilla and heat until very warm without boiling or simmering.
Serve immediately, taking care that the chocolate will be hot! Add marshmallows as a garnish.
You can transfer this hot cocoa to a slow cooker to keep warm.