Add the soy sauce, garlic, 1 tablespoon oil, brown sugar, rice vinegar, sesame oil, chili oil, ginger powder, crushed red pepper (if using), and green onions to a jar or medium bowl. Cover tightly and shake vigorously or stir well with a fork or small whisk. If you’re not using the sauce right away refrigerate until ready to use.
Add the chicken pieces to a large, zip top bag. Sprinkle with the cornstarch. Close the bag without removing the air. Double check that it properly closed and shake well to coat the chicken pieces.
Heat the remaining 1 tablespoon of oil over medium heat in a large saute pan or skillet (preferably non-stick). When the oil is hot (but not smoking) add the chicken pieces and arrange it in a single layer. Cook it, undisturbed, for 5 minutes.
Use a spatula to flip the chicken in sections so that the browned side is up. Continue cooking for 5 minutes.
Add the sauce to the pan, stir well. When the sauce starts to simmer, lower the heat to medium-low and cover.
Cook for 3-5 minutes or until the chicken is cooked through. Stir occasionally.
The cooking time will depend on the size of the chicken pieces. Check the largest, thickest piece. There should be no pink in the center and the internal temperature needs to be at least 165°F.
Serve the chicken with rice, noodles, or steamed vegetables, and garnish with sesame seeds and extra green onions, if desired.