Combine milk with spices and sugar in a large heavy saucepan. Cook over medium heat, stirring to dissolve sugar, until milk just starts to bubble around the edges (scalding). Lower heat and cook 10 minutes. Remove from heat and allow to steep 10 additional minutes, then strain into another large heavy pot. Heat gently, then add chocolate and rum (add rum off flame). Whisk briskly 5 minutes over low flame until chocolate is completely melted.