In a medium saucepan, combine the milk, chocolate, cocoa powder, sugar, salt, cinnamon, and espresso powder.
Heat over medium, whisking frequently, until chocolate melts and mixture is smooth. Do not let the mixture boil and whisk often.
Remove from the heat and stir in the vanilla. Taste and add additional sugar 1 teaspoon at a time until your desired flavor is reached (if you are using chocolate closer to 60%, you may want one or 2 teaspoons more). Enjoy immediately, or transfer to a slow cooker to keep warm.
TO STORE: After cooling to room temperature, store in a covered container in fridge. Hot chocolate is best served within a day.
TO REHEAT: Warm in microwave or in saucepan on the stove until steaming.