Cut the root of the lemongrass stalks and peel away any tough, dry outer leaves. Cut the stalk into 2-3 inch pieces and bruise with a pestle or another heavy object (the lemongrass should split apart a in the middle).
Bruise the chilies to expose the seeds.
Preheat an unoiled wok to the highest possible heat for a few minutes. Add the oil (it will smoke severely, which is the point), and the corn, stir frying for 1-2 minutes. The corn will pick up some charred black marks.
Quickly stir fry the shallots and garlic for about 1 minute before knocking the heat down once again to medium low. Add the coconut milk, water, soy sauce, makrut lime leaves, bruised lemongrass and chillies, coriander roots, palm sugar, and white pepper.
Simmer for 10-15 minutes. Remove the lemongrass and makrut lime leaves before pureeing the soup in a blender (allow the liquid to cool first if your blender is glass). If you don't want an overly spicy soup, also remove the crushed chilli (plenty of the heat will have been imparted).
Season with salt to taste and serve with a squeeze of fresh lime juice.