Mix together soy sauce, sugar, vinegar, cornstarch, cold water, ginger, garlic, and black pepper in a bowl until marinade is well combined. Add chicken breasts and stir to coat.

Pour marinade and chicken into a 9x9-inch square baking dish. Cover and marinate chicken in the refrigerator for 1 hour.

Preheat the oven to 325 degrees F (165 degrees C).

Bake chicken in marinade in the preheated oven until meat is no longer pink in the center and juices run clear, about 40 minutes, turning halfway through cooking.