Put the stock and coconut milk into a soup pot and add lemongrass, galangal, scallions, cilantro, and lime leaf. When making a large batch, put all the seasonings into a nut milk bag and "steep" it in the broth like a huge tea bag, so you don't have to strain a huge, heavy pot later.
Bring to a simmer to infuse the herbs into the broth for about 10-15 minutes, or to taste. Do not boil!
Strain broth through a fine mesh sieve or simply remove the nut milk bag.
Add the sugar to the broth, if using, and bring to a low simmer of about 160-180 degrees. Do not boil!
Add chicken, mushrooms, corn, and other veggies, and simmer until the chicken is completely cooked and the mushrooms are tender, about 5-10 minutes.
Once the chicken is cooked through, throw in the smashed chilies and remove the pot from heat immediately.
Add the juice of 2 limes and the fish sauce, stir, and taste. Add more lime juice and fish sauce, if necessary. The soup should be predominantly sour, followed by salty/umami.
Garnish with cilantro leaves, lime juice and hot sauce, if using.