Preheat the oven to 350°F.
Add the chicken breast chunks to the bowl. Season with salt and freshly ground pepper to taste. Then, transfer seasoned chicken breast chunks and cornstarch to a plastic bag. Close the bag and shake to coat completely. Set aside.
In a large skillet, heat vegetable oil over medium-high heat. While vegetable oil is heating, beat eggs in a small bowl or shallow pie plate.
When vegetable oil is heated, dip each chicken breast chunk in the beaten egg. Then, transfer the coated chicken breast chunks to the skillet in a single layer. Cook over medium-high heat until golden on both sides (about 1 minute on each side). The chicken breast chunks will still be raw, but that's okay. You will continue to bake them in the oven. Repeat with the remaining chicken breast chunks. Add additional vegetable oil, if needed.
Grease a 9x13 inch baking pan with non-stick cooking spray. Then, transfer the fried chicken breast chunks to the prepared baking pan in a single layer. Set aside. If all of the chicken breast chunks don't fit on the baking pan in a single layer, you can use one more baking pan or bake them in batches.
Prepare the sweet and sour sauce in a small bowl. Mix together granulated sugar, ketchup, rice vinegar, soy sauce, salt, and garlic powder. Pour over the fried chicken breast chunks. Bake in the preheated oven for 50-60 minutes until chicken is golden brown and the sweet and sour sauce is thick. For a better result, turn the chicken breast chunks every 15-20 minutes. Enjoy!