Heat the peanut oil in a large frying pan over medium heat. Add the chicken mince and cook, stirring constantly to separate the mince, for 4-5 minutes or until cooked. (See microwave tip.) Spoon the chicken and pan juices into a heat-resistant bowl, cover and place in the fridge for 30 minutes, stirring occasionally, or until cool. Meanwhile, place the noodles into a heat-resistant bowl, pour over boiling water to cover and set aside for 25-30 minutes or until tender. Drain. Combine the cooled chicken, water chestnuts, chillies, green shallots, lime juice and fish sauce in a large bowl. Stir in the noodles and mint. Serve with the lettuce leaves.