Put masa into a large saucepan. Toast over medium heat until it smells nutty, then whisk in water. Bring to a simmer and add milk, chocolate, honey, and salt. Stir until chocolate is melted, about 1 minute. Add cinnamon stick.
Return to a simmer and lower heat to low. Continue to simmer gently, whisking constantly, for 5 minutes. Discard cinnamon stick and add vanilla. Thin with additional water, as needed, to create a thick-yet-drinkable hot beverage (the exact consistency is a matter of personal taste), reheating as necessary. Taste, adding more sugar or salt if desired. Puree with immersion blender until frothy.
