Cut the beef into 1/3-inch cubes. Place the beef, peanuts, and thin coconut milk in a large covered saucepan and boil gently over medium heat for 30 minutes.
Peel the potato and cut it into 1/3-inch cubes. Add it to the saucepan, stir, and cook the mixture, covered, for 15 minutes more. Remove the mixture from the heat and set aside.
Bring the thick coconut milk to a boil in a saucepan, and remove it from the heat.
Heat a wok (or frying pan), and add 1/4 cup of the thick coconut milk and the Mussaman Curry. Stir the mixture over moderate heat until it is thick and pale tan. Add the cardamom seeds. Add the rest of the thick coconut milk 1/4 cup at a time, stiffing it over high heat until the sauce becomes slightly thick after each addition.
Add this mixture to the beef mixture, and bring it to a boil. Add the fish sauce, sugar, and tamarind solution or concentrate (If you have used concentrate, add 1/2 cup warm water and stir well to mix).
This dish may be prepared a day in advance and stored, covered, in the refrigerator. It actually improves in flavor if prepared ahead. Warm without boiling.