In a heavy-bottom saucepan, heat the milk over medium heat until it is just about to boil, about 10 minutes. Remove from heat, stir in the salt, cinnamon and cardamom and set aside to steep for a half hour or so. Meanwhile, chop or break up the chocolate and place in a bowl. Reheat the milk until it just comes to a boil, then remove from heat and strain through a fine mesh sieve over the chocolate. Stir until the chocolate is melted and the mixture is smooth. Taste, and adjust sweetness with sugar if desired. Serve the soup in small bowls; the soup can also be made ahead of time, chilled and then reheated before serving.