Coat the chicken in half the curry powder, the sugar and a good pinch of sea salt. Set aside for 30 minutes.
Heat the oil in a wok and add the shallots, garlic and lemongrass. Stir for a couple of minutes until softened then add the remaining curry powder, chilli flakes and the chicken. Stir fry for 2 minutes more.
Add the sweet potato, 1 tbsp fish sauce, the coconut milk and about 75 ml water. Bring to the boil then simmer for 15-20 minutes until the chicken is cooked through. Taste and add more fish sauce if needed.