Pork Potstickers

Toss the finely chopped napa cabbage with 1/2 tsp salt in a colander and let it sit for 10 minutes to draw out excess water. Squeeze the cabbage firmly with your hands to remove as much liquid as possible; this prevents soggy filling that leaks through the wrappers.
In a large bowl, combine the ground pork, squeezed cabbage, green onions, ginger, garlic, soy sauce, sesame oil, Shaoxing wine, and sugar. Mix with a spoon or your hand in one direction for about 1 minute until the filling looks slightly sticky and well combined.
Set up a small bowl of water for sealing. Place a dumpling wrapper in your palm and spoon about 1 tsp of filling into the center; do not overfill or it will be hard to seal.
Dip a finger in the water and moisten the edge of half the wrapper. Fold the wrapper in half over the filling and pinch the center point to seal.
Starting from the center, make 3-4 small pleats along one side of the edge, pressing each pleat firmly against the flat back edge to seal tightly, leaving no gaps. Repeat with remaining wrappers and filling.
Heat 1 1/2 tbsp of the vegetable oil in a large nonstick skillet with a lid over medium heat.
Arrange half the dumplings in the skillet in a single layer, flat side down, not touching each other; overcrowding will make them stick together and steam unevenly.
Fry undisturbed for 2 minutes until the bottoms are golden brown; check by lifting one gently with a spatula.
Carefully pour in 1/4 cup water and immediately cover the skillet with a lid; the water will bubble and steam vigorously, so stand back.
Steam for 5-6 minutes covered, until the water has mostly evaporated and the wrappers look translucent and the pork filling is fully cooked (internal temperature 160F/71C).
Remove the lid and let any remaining water cook off for 1-2 minutes so the bottoms re-crisp; listen for the sizzle to return.
Slide the dumplings out onto a plate, crispy-side up, and repeat with the remaining dumplings and oil and water.
Mix the soy sauce and black vinegar in a small bowl for dipping and serve the potstickers hot.

Squeezing all the water out of the salted cabbage is the single most important step to prevent watery, wrapper-splitting filling.

Adding water to the hot oiled pan and immediately covering it creates the steam that cooks the filling through while the bottom stays frying and crisping underneath.