Combine first 3 ingredients in an 8- x 10-inch piece of cheesecloth; pull edges together, and tie to secure.
Heat milk and spice bundle in a 3-quart heavy saucepan over medium-low heat. Bring just to a simmer. Remove from heat, cover, and steep 30 minutes.
Remove spice bundle from milk. Heat milk mixture over medium-low heat until hot. Add brown sugar; cook, stirring constantly, until sugar melts. Add chocolate, stirring until chocolate melts. Stir in vanilla. Serve warm; garnish, if desired.