Soak noodles: Soak dangmyeon (mung bean noodles) in warm water until softened, about 15 minutes. Drain and chop finely.
Make filling: In a large bowl, combine ground pork, chopped napa cabbage, chives, chopped dangmyeon, garlic, ginger, soy sauce, sesame oil, salt, and pepper. Mix with your hands until well blended.
Set up: Keep wrappers covered with a damp towel so they don’t dry out. Place a small bowl of water nearby for sealing.
Fill and seal: Place a wrapper in your palm and add about 1 tablespoon filling in the center. Moisten the edges with water, fold in half, press out air, and seal. (Pleat if you like for a traditional look.)
Pan-fry (gun-mandu): Heat 1–2 tablespoons oil in a large skillet over medium-high. Add dumplings in a single layer and cook 2–3 minutes until the bottoms are golden.
Steam in pan: Carefully add about 1/3 cup water, cover, and steam 6–7 minutes until dumplings are cooked through and water evaporates. Uncover and cook 1 minute more to re-crisp the bottoms.
Or steam/boil: Steam in a lined steamer basket 12–15 minutes, or boil in gently rolling water 6–8 minutes until they float, then cook 2 minutes more.
Dipping sauce: Stir together soy sauce, rice vinegar, sesame oil, and optional gochugaru. Serve dumplings hot with dipping sauce.