It's called "iron egg" because it's so tough. These chewy little eggs, dyed black from long braising in soy sauce, are a highly addictive Taiwanese food.
Often made from quails' eggs, the protein balls are cooked for hours in soy sauce then air-dried. The process is repeated over several days until the snacks become tough and acquire the desired chewiness.
Seaside A-Po, 151-1, Jhongjheng Road, Tamshui; A-Po, 135-1, Jhongjheng Road, Tamshui