Start by cutting chicken breasts into chunks. Mix together soy sauce, cornstarch and water. Add the chicken and stir to coat. Allow the chicken to marinate for 5 minutes.
While it is marinating, prepare the orange sauce so that it is ready when you need it later. Mix together oil, orange juice and barbecue sauce.
Heat 2 tablespoons of oil in a large skillet. Remove the chicken from the marinade with a slotted spoon and carefully add it to the skillet. Stir fry the chicken, over medium-high heat, for 5-7 minutes until golden and cooked through.
Add the orange sauce to the chicken and cook for 4-6 minutes, stirring constantly until the sauce has thickened.
Top with sesame seeds and sliced green onions, if desired.