1. Remove fat from chicken and rub chicken with 2 tsp salt. Rinse chicken under cold water and place on a rack to drain.
2. In a pot large enough to fit the chicken, bring about 4.5 litres/3 pints cold water, ginger, garlic and 2 tsp salt to a boil over high heat, covered. Carefully add the chicken, breast-side up, adding more boiling water if necessary to cover the chicken.
3. Return to a rolling boil and boil vigorously over high heat, uncovered for 5 minutes, removing any greasy film from the surface. Cover the pot and let it sit off the heat for 20 minutes.
4. Uncover the pot and bring to a boil over high heat. When the broth returns to a rolling boil, boil vigorously, uncovered for 3 to 4 minutes. Remove from heat. Using 2 heavy spoons, carefully transfer the hot chicken to a colander in the sink. Make sure the chicken is done by testing it with a meat thermometer. It should register 77C when tested in the meatiest part of the thigh.
5. Slowly pour the iced water over the chicken in the colander, then transfer to a cutting board.
6. Rub the warm chicken with the sesame oil. Allow chicken to cool slightly before using a meat cleaver to chop through the bone into bite-sized pieces, reserving any chicken juices.
7. Place the spring onions, ginger and remaining 1? tsp salt in a heatproof dish. In a small pan, heat the vegetable oil over high heat until hot but not smoking. Carefully pour the hot oil over the spring onions and ginger. The oil will make a crackling sound.
8. Serve the chicken warm or at room temperature with the spring onion and and ginger sauce.