Combine half the coconut milk with the galangal, lemon grass, and lime leaves in a large saucepan and heat to boiling. Add the chicken, fish sauce, and sugar. Simmer for about 4 minutes, or until the chicken is cooked, and then add the remaining coconut milk. Heat just to boiling. Place the lime juice and chilli paste in a serving bowl and pour the soup over them. Garnish with cilantro leaves and crushed chilli peppers.