Coat the chicken pieces in half the curry powder, the sugar and a generous pinch of salt. Set aside for an hour if you have time.
Heat the oil in a wok and add the shallots, garlic and lemongrass. Stir fry for a couple of minutes until softened. Add the remaining curry powder, the chilli flakes and the chicken. Stir fry for a couple of minutes more.
Add the sweet potato, 1 tbsp of the fish sauce, the coconut milk and half a wine glass of water. Bring to the boil then simmer for 15-20 minutes until the chicken is cooked through.
Taste and add a little more fish sauce if needed. Scatter with coriander and served with the baguette or steamed rice.