In a small saucepan over low heat, combine the cornstarch, cold water, sugars, soy sauce, vinegar, garlic, ginger and ground black pepper. Let simmer, stirring frequently, until sauce thickens and bubbles.
Preheat oven to 425 degrees F (220 degrees C).
Line a 9x13 inch baking dish with foil and place chicken inside. Pour most of the sauce over the chicken, reserving about 1/2 cup. Cover the dish in foil.
Bake in the preheated oven for 30 minutes. Turn pieces over and add the remaining sauce and sprinkle with sesame seeds if desired.
Bake for another 20-30 minutes minutes, until no longer pink and juices run clear.