Cut chicken into large chunks (about 1½ -inch/4cm pieces). Add the cornstarch and water. Mix well until the chicken has absorbed the water and is evenly coated in the mixture. Then mix in 1 tablespoon of oil. This step will keep the chicken silky and moist during cooking.
Preheat the wok until it starts to smoke lightly (this is to prevent sticking). Reduce the heat to low. Wait a minute or so to lower the wok temperature, then add 3 tablespoons of the oil (or three quarters of the total, if you’ve scaled the recipe up/down) to your wok along with the sugar. Let the sugar melt, stirring with your spatula. The rock sugar takes longer to melt than granulated sugar—take care not to let it burn. As you do so, stir the oil evenly around the wok.
Increase the heat to medium-high, and add the chicken pieces in a single layer. Don’t stir until the chicken pieces are lightly browned (about 1 minute)—this is so the chicken has a chance to sear, and to prevent sticking. Flip the chicken pieces to lightly brown them on the other side—1 minute.
Reduce the heat to medium. Add the ginger, Shaoxing wine, light soy sauce, dark soy sauce and water. Cover and simmer over medium heat for 8-10 minutes.
If there is still a lot of visible liquid at the end of the cooking time, uncover the wok, turn up the heat, and stir continuously until the liquid reduces to a rich sauce that coats the chicken. This may take 1-2 minutes or a few more, depending on how hot your stove gets and how close your wok is to the heat source. Mix in the chopped scallion and serve!