Prepare the pig feet gelatin a day ahead by boiling all the ingredients together in a pot. Skim any scum that forms on top of the broth and slowly simmer for about an hour or until the liquid is reduced to about one cup. Drain and filter the broth through a fine sieve. Refrigerate the broth overnight and it will solidify. Alternatively a chicken broth gelatin can also be used. Melt one packet (1/4 ounce) of gelatin in one cup of hot Chinese chicken stock and refrigerate overnight.
In a bowl mix the flour and oil then add the boiling water. Mix quickly with a pair of wooden chopsticks or a wooden spoon. When the mixture is cool to the touch start kneading the dough. If the dough is too dry add a little bit of extra warm water. Knead the dough until it is elastic and smooth. Set aside to let it rest.
Mix all the stuffing ingredients together and set aside. When making the dumplings work with half of the stuffing at a time while keeping the rest in the refrigerator. If the stuffing is not kept cool the gelatin will melt.
Divide the dough into two equal portions. Roll each portion into a cylinder about 1/2 inch in diameter. Cut the cylinder into pieces about 1/2 inch wide. Roll the pieces into balls. Roll the balls into round wrappers and fill with about 1 tablespoon of stuffing. Cup the wrapper in one hand, then with the other hand make small pleats along the edge all the way around and over the stuffing to form a seal.
Steam the dumplings in a steamer for no more than five minutes. Serve the dumplings immediately with thinly shredded ginger and black vinegar.