Cut chicken into roughly 1-inch pieces.
Preheat the oven to about 325 degrees Fahrenheit. Coat a 9 x 13 baking dish with nonstick cooking spray or lightly oil it.
Combine ketchup, garlic powder, sugar, vinegar, and soy sauce in a bowl; place aside.
Season chicken with pepper and salt. Add cornstarch and toss lightly to coat. Dip each chicken piece into the eggs one at a time.
In a big saucepan, heat the vegetable oil. Cook the chicken for 1-2 minutes, or until golden brown. Take out to a dish lined with kitchen paper towels to drain any extra oil.
Transfer the chicken to the prepared baking pan. Toss with prepared sauce mixture—Bake for around 50 minutes, or until the sauce thickens. Turn the chicken over after 15 minutes to ensure equal cooking.
Garnish with green onion for added flavor and color. Serve immediately and enjoy!