In a mixing bowl, combine peanut butter, soy sauce, dark mushroom soy sauce, garlic chili oil, gochugaru, and rice vinegar. Whisk until smooth, adding water as needed.

Bring a large pot of salted water to a boil. Add noodles and cook according to package instructions (about 4-6 minutes). Drain and reserve 1/4 cup of cooking water.

Return the drained noodles to the pot over low heat, pour the sauce over, and toss to coat. Add reserved noodle water gradually if the sauce is too thick.

Transfer to serving bowls and top with chopped spring onions, coriander, sesame seeds, and a drizzle of sesame oil. Serve immediately.