Heat oil in a large wok (or other large heavy bottom pan) over medium heat, add the curry paste and cook briefly until fragrant. Add the reserved coconut cream and fry until it begins to separate, stirring constantly. Add the boiled beef and peanuts in its broth and remainder of coconut milk, fish sauce, palm sugar, tamarind, cinnamon stick, potatoes and onion. Bring to a boil. Reduce heat to low and cook 10 minutes.
Adjust seasoning with fish sauce, palm sugar and tamarind water to taste. Massaman curry should be slightly sweet with a very subtle sour note from the tamarind. Serve with pickled ginger or cucumber relish as condiments.
Serves 4. This curry is even better the next day, reheated, so make double portion and freeze any left-overs.
*Learn how to properly dry roast spices for Thai cooking from Thai cooking instructor Kasma Loha-Unchit.