A true denizen of northern China might scoff at commercial wrappers for dumplings, but really they are so much easier, even for a small batch like this (and before you ask, yes, I can roll my own - albeit slowly). The best and cheapest places to buy wrappers is at shops that make fresh or dried noodles.
The skins are available round or square (buy round for this recipe), and often, in two thicknesses. Buy the thicker ones, if you have the choice - they are still quite thin, but the texture is a little more resilient and there's less chance of the skin tearing during cooking.
Some recipes suggest making a whole batch of dumplings at once and freezing any uncooked ones. I find it easier to just freeze the excess filling, packed into an airtight container, and then fill the skins next time you want dumplings. It takes less room in the freezer and you don't have to worry about the dumplings breaking - the skins can become brittle when frozen.
I mix some iced water into the filling, because it makes the dumplings juicier. But if you have and unsalted meat broth in your fridge or pantry, use that instead - just make sure it's cold. If you have time, chill the filling until it is very cold - another trick to juicy dumplings.
Cut the cabbage leaves lengthwise into thin strips, then slice them as thinly as possible. Put the pieces in a bowl, sprinkle with five grams (1 tsp) of salt and combine. Leave for about 10 minutes, then drain the cabbage in a colander. Spread the pieces on a clean, dry dishcloth, roll tightly and squeeze out the excess moisture.
Put the lamb in a bowl and add the soy sauce, rice wine, 5 grams (1 tsp) of sea salt, chilli flakes, sugar, pepper, sesame oil and corn starch. Mix thoroughly.
Put the cumin seeds in an unoiled pan placed over a low-medium flame. Shake the pan almost constantly until the cumin seeds are lightly toasted. Cool to room temperature, then crush the seeds in a mortar (I use a Japanese suribachi) until they are roughly ground – it’s fine if some are whole.
Finely mince the ginger, spring onions, chives, garlic and water chestnuts.
Add the cabbage, cumin, ginger, spring onion, chives, garlic and water chestnut to the lamb mixture and mix until the ingredients are evenly distributed.
Pour 60ml (¼ cup) of iced water into the bowl. Mix the filling vigorously with your hand, stirring in one direction until you see long, white protein threads forming.
Shape some of the filling into a flat patty. Heat a skillet and, when it’s hot, pan-fry the patty until cooked. Taste it to check the seasonings, then correct, if necessary.
Transfer the rest of the filling to a flat dish and refrigerate for at least 30 minutes (although longer is better).
Fill a small bowl with water. Put some of the dumpling wrappers on a small plate, keeping the others covered.
Place one of the wrappers in the palm of your left hand (if you are right-handed). Use a spoon to scoop up some of the filling into an oval-shaped ball and place it on the wrapper. Use a fingertip to lightly dampen slightly more than half of the perimeter of the wrapper, then fold up the other side to enclose the filling. Pleat the wrapper as you like: my easy method is to firmly pinch the centre of the two sides together, then use the forefingers of each hand to form a single pleat on each side, equidistant from the centre, and shaping the dumpling into a crescent. Place the dumplings on a tray, flattening their bottoms slightly. Chill the finished dumplings while shaping the others.
To cook the dumplings, bring a large pot of water to the boil over a high flame. Add some of the dumplings, leaving plenty of room for them to swim. When the water comes back to the boil, add about 250ml (1 cup) of cool water. Let the water boil again, then add another 250ml (1 cup) cool water. (This cooks the dumplings gently, lessening the chance of them falling apart as the water boils.) When the water comes to the boil the third time, the dumplings should be floating on the surface, which indicates they are cooked. If necessary, add more cool water and bring to the boil one more time.
Use a slottted ladle or skimmer to scoop the dumplings out of the water and drain them briefly before dividing them into individual bowls.
If you want to pan-fry the dumplings, chill them until very cold. Heat a skillet (preferably cast-iron) over a medium-high flame and oil it liberally.
Lay the dumplings in the skillet so they are just touching each other and pan-fry them until their bottoms are lightly browned.
Pour in about 60ml (¼ cup) of water, then immediately cover the pan and turn the heat to low. Let the dumplings cook for three to five minutes, or until done. If the water evaporates too quickly, drizzle in a little more.
Remove the lid and let the dumplings pan-fry until their bottoms are medium brown. Lift them out of the pan and place them brown-side up on a serving dish.
Let each diner mix the ingredients for their own dipping sauce for the dumplings.