Return to a simmer and reduce the heat to low. Continue to simmer gently, whisking constantly, for about 5 minutes, until the drink is thickened and infused with the flavorings. Discard the whole spices and zest, and whisk in the chocolate. Thin with additional milk or water, as needed, to create a too-thin-to-eat-with-a-spoon, thicker-than-your-average drink consistency. Taste, and adjust the sweetness and salt to your preference. Ladle into mugs, garnish as desired, and serve. You can refrigerate this for later reheating in a tightly lidded jar, but it definitely should be served warm. It will keep up to 3 days refrigerated; simply thin with a little more milk or water and whisk it over low to medium heat to serve.