In a medium bowl, toss together the cornstarch, chicken, egg white, and salt. Set aside to marinate for 10 minutes.
Heat a large wok over high heat until it starts to smoke. Add 2 tablespoons of the oil and swirl to coat the bottom and sides of the wok. Add the chicken and cook, stirring constantly, until opaque, 1–2 minutes. Transfer the chicken along with any juices to a bowl and set aside.
Return the wok to the stove, add the remaining oil, and swirl as above. Add the ginger and leek and cook, stirring constantly, until fragrant, about 30 seconds. Add the celery and cook, stirring frequently, until crisp-tender, about 1 minute. Add the reserved chicken and juices back to the wok, along with the soy sauce, and cook, stirring frequently, until the chicken is cooked through, 1–2 minutes more.
