In a bowl, combine water, soy sauce, brown sugar, garlic powder, and ground ginger. Whisk until well mixed.
Reserve half of the sauce mixture. Marinate chicken pieces in the remaining half of the sauce for 2-3 hours, turning once halfway through.
Add cornstarch to the reserved sauce mixture and whisk to combine. Bring to a boil over medium-high heat, stirring constantly, until the mixture thickens slightly. Remove from heat and cool to room temperature.
Drain the marinade from the chicken, reserving 2 tablespoons of the used marinade. Discard the remaining marinade.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken pieces and stir-fry until almost cooked through.
Add the reserved 2 tablespoons of marinade to the pan and continue stir-frying until chicken is fully cooked and glossed with the sauce.
Serve immediately over hot steamed rice with the cooled teriyaki sauce on the side.