These dumplings are similar to the Japanese gyoza – you could even fry them after steaming if you wanted the extra crunch. Sesame salt is a common seasoning in Korean cooking – make extra to have on hand for seasoning other dishes too.
For sesame salt, dry-roast sesame seeds in a frying pan over medium-high heat until roasted (2-3 minutes). Cool slightly, set aside 1 tsp for dipping sauce, then pound remainder with salt in a mortar and pestle until finely ground.
Combine pork, tofu, kimchi, chives, spring onion, sesame oil and a large pinch of freshly ground pepper in a bowl and season to taste with sesame salt (about 1 tsp). Set aside.
Lay a few wrappers on a work surface, place a teaspoonful of pork mixture in centre of each, then brush edges with a little water. Fold in half to form a semicircle, then trim edges with a 7cm-diameter cutter. Pleat edges and set aside on a lightly floured tray. Repeat with remaining wrappers and filling. Cook mandu in batches in boiling water over medium-high heat until cooked through (2-4 minutes). Drain and keep warm.
Meanwhile, for dipping sauce, combine ingredients and reserved roasted sesame seeds in a bowl. Serve with mandu, kimchi and extra sesame salt.