Crush the seeds of your black cardamom with a mortal and pestle or spice grinder. Prep your onions and the other spices.
Heat your oil and fry the cardamom seeds to release their flavor in the oil for a few moments. Add the asafoetida and tumeric and fry together for under one minute. Add the onions, and sauté until you have a the beginning of an onion masala / thick gravy.
Add your eggplant and salt, and thoroughly stir the ingredients to make sure that the eggplant pieces are coated in masala. You will want to stir for about ten minutes. The eggplant will become jammy, particularly if you add a lid to your cooking pot for just a minute or so towards the end.
Stir in the mango powder and cilantro towards the end, and remove the pot from the stove.