Pick the stem off the Kashmiri chili and place the Kashmiri chilis in a spice grinder and grind into a powder. Rub the pork pieces with half of the kashmiri chili powder, cumin, turmeric, and salt and let it marinade for 30 minutes (or overnight if you prefer)
Preheat a large heavy-bottomed pot with oil. Add in the mustard seeds and fry until they start popping. Add in the cardamom pods, cloves, cinnamon stick and saute for about 1 minute. Add in the onion and saute until they are soft, about 1 minute. Add in the garlic and saute for another 30 seconds. Add in the kashmiri chili powder you made earlier. Add in the pork and cook for another 3 minutes. Pour in the water, cider vinegar and tamarind juice and the paste. Bring to a boil and then lower the heat, cover and let it simmer for 1 hour or until the pork is really tender. Add the potatoes (if using) in the last 30 minutes. Stir in the sugar. Have a taste, it should be spicy, savory, tangy and lightly sweet. If you make this for the next day. Let it cool down and store in the refrigerator and reheat it the next day. It taste so much more better the next day.