Cut the chicken into bite-sized pieces. In a mixing bowl, combine the chicken with the marinade ingredients (cornstarch, soy sauce, sesame oil, and sugar or honey). Mix well and let it marinate for 30 minutes.
Heat a cooking pan or wok over medium-high heat and add the cooking oil. Once the oil is hot, add the garlic and marinated chicken, including the marinade. Stir fry for about 2 minutes, or until the chicken is no longer translucent. Remove the chicken from the pan and set it aside on a plate or in a bowl, leaving the sauce in the pan.
If the pan is too dry, add a little more cooking oil. Add the zucchini and stir fry for about 5 minutes, or until the zucchini starts to become tender.
Return the chicken to the pan, add the oyster sauce, and sprinkle with ground black pepper if desired. Stir fry everything together for another 2-3 minutes, or until the chicken is fully cooked and the zucchini is tender.
Remove from heat and serve immediately.