Bring the chicken stock to a boil, and then add the galangal, lime leaves and lemongrass and boil for 30 seconds. Add the chicken and simmer
until cooked, 1 to 2 minutes. Stir in the coconut milk, mushrooms and chiles. Return to a boil, then add the fish sauce, lime juice, sugar and sweet chili. Stir to combine. Drop in the truffle oil and garnish with coriander leaves. Serve hot.