Sift the flour into a large mixing bowl. Dissolve the sugar in the hot water, add the yeast, and mix well; leave to stand for 10 minutes. Add the yeast to the flour and mix in the metled sortending or lard. Mix well, remove from bowl and knead for about 3 minutes. Shape into a long sausage shape and cover with a cloth. Blend the sauce ingredients together and set aside. Heat the oil in a wok and fry the diced cha shao, the leek and the ginger over a high heat for a minute. Remove the leek and giner and discard; pour in the sauce and then add the cornstarch and water to thicken. Stir and cook until the mixture is smooth and homogenous. Leave to cool. Cut the roll of dough into 24 pieces, flattening each piece with your fingers and shaping into a disk. Place a tablespoon of filling in the center of each round of dough and enclose, pinching the dough closed with the fingers. Place a piece of waxed paper or foil under each dumpling and leave it to rise for 10 minutes. Cook in a bamboo steamer for about 10 minutes taking care to leave each dumpling enough space to expand. Do not open the steamer while the dumplings are cooking. Serve warm.