In a small saucepan over low heat, combine the cornstarch, cold water, sugar, soy sauce, vinegar, garlic, and ground black pepper. Let simmer, stirring frequently, until sauce thickens and bubbles.
Preheat oven to 425 degrees F (220 degrees C).
Using scissors, remove any pieces of fat from each chicken thigh. Place chicken pieces in a 9x13 inch baking dish sprayed with Pam cooking spray. Pour 1/2 half of the sauce over the chicken, brushing each piece so it is coated.
Bake in the preheated oven for 30 minutes. Turn pieces over, pour on remaining sauce, and bake for an additional 30 minutes, or until chicken juices run clear.