Preheat oven to 325°F (163°C).
Season chicken pieces with salt and pepper.
Coat each piece of chicken in cornstarch, then dip in beaten eggs.
Heat oil in a large skillet over medium-high heat. Sear chicken on each side until golden but not cooked through.
Transfer chicken to a 9x13-inch baking dish.
In a medium bowl, whisk together sugar, ketchup, vinegar, soy sauce, and garlic powder.
Pour sauce over chicken and toss to coat evenly.
Bake uncovered for 1 hour, turning the chicken every 15 minutes to keep it coated in sauce.