Set a large pot of water on the stovetop over high heat. Bring to a boil. Once boiling, cook the pasta according to the package instructions. Then drain off the boiling water and set the cooked pasta aside.
Meanwhile, set another large pot over medium heat. Add 1 cup coconut milk. Bring to a boil and cook the coconut milk down into a thick paste that starts to separate.
Once the coconut milk separates, add the curry paste and tumeric. Cook the mixture 2-3 minutes. Then add in the chopped chicken pieces. Stir and cook for 5 minutes.
Once the chicken looks mostly cooked, pour in the remaining coconut milk, chicken broth, and fish sauce. Bring to a boil, then lower the heat and simmer for 10 minutes.
While the soup is cooking, chop/prep the garnishes. Get Fancy: If you want to top the Khao Soi in the traditional Chiang Mai style with fried noodles, place 1-2 inches of vegetable oil in a separate pot. Once it is smoking hot, drop 2-3 extra egg noodle nests in the oil and fry for approximately 20 seconds per side. Remove and drain. Then separate into 6 chunks for the garnish.
Once the soup base has simmered for 10 minutes, stir in the cooked pasta and lime juice. Stir and simmer for 1 minute to warm the noodles.
Ladel the soup into bowls. Top with chopped scallions, cilantro, fried noodles, and lime wedges if desired.
Protein Options: If you are not a fan of chicken, Kao Soi can be made with shrimp, thinly sliced pork, or thinly sliced beef.