Preheat oven to 425 degrees F. Line a 9x13 baking dish with foil and coat with nonstick cooking spray.
Place chicken pieces in the baking dish. Season with a bit of salt and pepper on both sides, to taste.
In a small saucepan, whisk together the water with the cornstarch until cornstarch is dissolved, to create a slurry. Whisk in all the remaining sauce ingredients until smooth and incorporated. Bring to a simmer, stirring it frequently to prevent overflow and scorching. Right when it boils, reduce heat to a low bubble for 3-4 minutes until the sauce has thickened, stirring occasionally. Remove the sauce from the heat.
Baste chicken thighs with the sauce. Turn thighs over, bone side up, and brush again.
Transfer to the oven and cook for 25 minutes, uncovered, basting with the sauce every 8-10 minutes. Turn chicken pieces over, and bake an additional 25 minutes, uncovered, basting every 8-10 minutes. (Or until no longer pink, juices run clear, and an internal temperate of 165 degrees F is reached.)
Thighs should have a gorgeous, glossy sticky amber-brown coating.
Sprinkle with diced scallions and serve with some of the pan sauce over cooked white rice and roasted snap peas or broccoli on the side.
NOTE: Mirin is a sweet Japanese rice wine, which is similar to sake, but with less alcohol and more sugar. It's available at all Asian markets and most common grocery stores. If you can't find it, a good substitute would be dry sherry mixed with a bit of granulated sugar. (Avoid rice wine vinegar, which would give the sauce an acidic taste.)
For information on using chicken breasts instead, helpful tips, and proper storage, please refer to the full article.