Add the chicken pieces and soy sauce in a bowl, and let it marinate for 10 minutes.
Add cornstarch to the chicken and toss to coat evenly.
Heat vegetable oil in a large pan over medium-high heat. Add the chicken and sear until golden brown on all sides.
Remove the chicken from the pan and set aside.
In the same pan, add garlic and ginger, sauté briefly until fragrant.
Add water, hoisen sauce, and brown sugar to the pan. Stir and bring to a simmer.
Return the chicken to the pan, and toss it in the sauce until heated through and coated well.
Mix in green onions, and cook for an additional minute.
Serve hot with rice or noodles.