In a small bowl, whisk together the broth, soy sauce, brown sugar, vinegar, sesame oil, ginger and red pepper flakes. Set aside
Place the cornstarch in a large bowl and stir in the salt and pepper. Add the chicken and toss to coat well.
Heat half of the vegetable oil in a large skillet over medium high heat. Add half the chicken, making sure it is in a single layer and cook without stirring for 3 minutes, until the bottom is browned. Turn the chicken (tongs are best for this) and let sit in the pan so that another side browns. Repeat until the chicken is cooked through and browned on as many sides as possible; not all of the sides need to brown, about 10 minutes in all. Transfer to a plate. Add the rest of the oil and cook the rest of the chicken the same way. Add the second batch of the chicken to the first batch on the plate, pour off any excess oil from the pan.
Return the cooked chicken to the pan and stir in the garlic and the scallions. Whisk the sauce again to blend and pour it into the pan. Cook, stirring frequently until the sauce thickens and coasts all of the chicken, about 1 minute. Serve the chicken over the rice sprinkled with sesame seeds if desired.
This recipe calls for chicken thighs, which is what is usually used in restaurants; it’s moister and more flavorful than chicken breasts, but you can use chicken breasts if that’s what you prefer. The cooking time with be slightly shorter for white meat (breasts) vs dark meat (thighs), so keep that in mind.