Boil the potato pieces for about 10 to 15 minutes, testing a piece with a fork before draining. The fork should slide through the center without resistance.

Transfer the thoroughly drained potatoes to a heatproof bowl, add the ½ teaspoon salt, and crush with a fork. Continue until the mash has no visible lumps that could interrupt a noodle strand.

Work the potato starch into the hot mash, then knead once the mixture feels comfortable to handle. The powder should gather into a cohesive mass as it absorbs the potato's heat and moisture.

Incorporate the warm water and knead until the dough absorbs it. A nearly smooth, moldable dough is the goal, rather than the elastic stretch of bread dough.

Bring a wide pot of water to a boil, arrange your rolling space nearby, and fill a large bowl with cold water. The pot should offer enough room for the ropes to enter the water without sharp bends.

Portion the dough into 14 equal pieces and roll each into a rope about ½ inch thick, keeping waiting pieces beneath a damp towel. A well-hydrated rope should hold together with a mostly smooth surface.

Lower a manageable batch of ropes completely into the boiling water and gently stir to release them from the bottom. Leave enough space for the delicate noodles to move without crowding or tangling.

Cook until the noodles rise to the surface, continue boiling for 1 additional minute, then lift them into the cold water and repeat with the remaining ropes. The cold bath firms the soft strands into slick, bouncy noodles.

Combine the soy sauce, black vinegar, gochugaru, sugar, remaining salt, garlic, and green onion in a heatproof bowl, then carefully pour over the oil heated in a small pan just until it begins to smoke. Expect vigorous sizzling, then stir once the bubbling subsides and the aromatics smell fragrant.

Toss the well-drained noodles in a clean serving bowl with the chili oil mixture and chopped cilantro, then serve warm. Each strand should carry a glossy coating of sauce without excess bath water diluting it.