Step 1Put flour in a large bowl. Make a well in the center and pour in one-half cup cold water. Slowly stir with chopsticks or a wooden spoon, moving from the center toward the rim, to work in all the flour. (Add more water by the teaspoon until most of flour is incorporated.) Keep stirring as a ragged, soft mass forms. Then use your fingers to gather and pat the dough together into a ball. Transfer to a work surface and knead for about 5 minutes, until smooth, fingertip-soft and slightly elastic. (You shouldn't need any additional flour on the work surface if the dough was properly made. Keep kneading, and after the first minute or two, the dough shouldn't stick to your fingers. If it does, work in a sprinkling of flour.) Press your finger into the dough; the dough should spring back, with a faint indentation remaining.
Step 2Transfer dough to a plastic bag and seal and let stand at least 10 minutes, covered.
Step 3If not using immediately, cover and refrigerate the dough until needed up to 1 day.
Step 1In a medium bowl, combine the beef, soy sauce, rice wine, sugar and salt. Use a fork, chopsticks or spatula to stir together the ingredients clockwise (do not stir in the opposite direction) until mixture is smooth like a paste. Add water in 3 stages, stirring clockwise until each addition is well-combined and liquid is emulsified. Stir in the oil in same manner.
Step 2Stir in leeks and ginger until well combined.
Step 3Use the filling immediately, or wrap and chill until ready to use, no more than 1 day.
Step 1To wrap the dumplings, divide the dough in half, keeping half wrapped. Transfer the remaining half to a work surface and gently roll into a 12-inch log about three-fourths inch in diameter.
Step 2Cut the log crosswise into 18 (two-thirds inch) pieces, rolling the log gently between cuts to keep it round. Generously dust the dough with flour to keep the pieces separate, and gently flatten each round piece into a circle using the palm of your hand.
Step 3Roll each piece into a 2 1/2 -inch round wrapper (this is easiest using an Asian dowel-style rolling pin), rolling the outer one-half inch edge very thin, like a tortilla (the center of the circle -- a little more than 1 inch -- will remain slightly thicker). Use 1 hand to roll the dowel back and forth while the other hand rotates the circle, like a steering wheel, to form the wrapper. Repeat with the remaining pieces, loosely covering each wrapper to keep it from drying out and dusting the wrappers to keep them separated. Repeat with the remaining dough half.
Step 4Form the dumplings: Hold a wrapper in a slightly cupped hand. Use chopsticks, a dinner knife or spoon to center about 2 teaspoons of filling on the wrapper, pressing down gently and keeping one-half to three-fourths inch of the dough clear on all sides; your hand will automatically close slightly.
Step 5Use the thumb of the hand cradling the dough to push down the filling while the fingers of the other hand pull up the edges and pinch the edges together. (Pinch the center of the dumpling first to seal, then pinch the remaining edges to enclose the filling completely.)
Step 6Position the dumpling so the sealed edge rests against the palm and along your first finger. Cup your other hand around the dumpling hand and bring your thumbs together and press firmly to seal the edge.
Step 7Place the dumpling, pinched side up, on a floured baking sheet or tray. Cover loosely with plastic wrap and refrigerate. Repeat with the remaining wrappers and filling, covering and refrigerating them as they are made.
Step 8Boil the dumplings: Bring a large pot of unsalted water to a boil over high heat. Add half of the dumplings and slowly stir, using the back of a ladle or large spoon to gently release any dumplings stuck to the bottom of the pot. When the water comes to a full boil, add a cup of tepid water to the pot, gently stir and return to a boil. Repeat twice. The dumplings are ready when they have boiled 3 times and float in the water.
Step 9Carefully lift the dumplings out with a slotted spoon into shallow bowls. Boil the remaining dumplings in a second batch. Save the dumpling water to serve with the end of the meal.
Step 10Serve the dumplings with black vinegar seasoned as desired with soy sauce and hot red pepper. Serve each guest a bowl of the cooking water at the end of the meal.