Preheat oven to 325F.
To prepare sauce, mix sugar, vinegar, ketchup, soy sauce, and garlic powder in a bowl.
Mix cornstarch, salt, and pepper in a bowl. Place eggs in a second bowl.
Heat oil in a skillet over medium-high heat.
In batches, coat chicken in cornstarch mixture, then coat in egg mixture, then add to the pan. Cook until golden brown, about 1-2 minutes. Transfer out of pan and place in a 9x13 baking dish.
Top chicken with sweet and sour sauce. Bake until sauce thickens, turning chicken every 15 minutes. The sauce may thicken faster or slower depending on your oven and baking dish, and will take between 15-45 minutes. Remove from oven as soon as the sauce is thick.
