In a small saucepan, combine the cornstarch with 1 cup of cold water. Whisk in the brown sugar, soy sauce, honey, ginger, and garlic powder. Bring to a simmer over medium heat and cook, stirring often, until the sauce has thickened enough to coat the back of a spoon, about 4 minutes. Reserve about ⅓ cup of the sauce for serving.
Pat the chicken dry with a paper towel and season all over with the salt. In a large bowl, combine the chicken with the remaining teriyaki sauce and toss until well-coated.
Preheat the grill on high. Transfer the chicken to the grill, allowing any excess to drip off. Reduce the heat to medium-high, cover the grill, and cook for 5 minutes, or until char marks appear. Flip, baste with any remaining sauce in the bowl, and cook, turning and basting occasionally, until the internal temperature reaches 165°F on an instant-read thermometer, 7 to 8 minutes more.
Transfer the chicken to a serving platter and drizzle with the reserved sauce. Let rest for about 5 minutes before serving.