Start by cutting the chicken into bite-sized pieces and seasoning lightly with salt and pepper. Place the chicken into a bag or bowl with cornstarch and coat thoroughly. This step creates the crispy outer layer that holds the sauce well during baking.
Heat oil in a large skillet over medium heat. Dip each cornstarch-coated piece of chicken into the beaten eggs, then place it into the hot skillet. Brown the chicken lightly on all sides without cooking it through.
Transfer the browned chicken into a baking dish in a single layer. In a bowl, whisk together sugar, ketchup, vinegar, soy sauce, and garlic salt until smooth. Pour the sauce evenly over the chicken so every piece is coated.
Bake uncovered at 325°F (165°C) for about one hour. Turn the chicken once or twice while baking so the sauce thickens evenly and coats each piece beautifully. The sauce should be glossy and slightly sticky when done.