Preheat your oven to 325°F and grease a 9×13-inch glass baking dish with non-stick cooking spray or a light wipe of oil.

Season your raw chicken breast cubes generously with salt and black pepper. Place the cornstarch into a wide, shallow bowl, and place the beaten eggs into a second separate bowl.

The Cornstarch Shield Coating: Dredge the chicken pieces completely in the cornstarch, shaking off any heavy excess, then drop them into the beaten eggs to coat. Pro Tip: Using pure cornstarch instead of flour is essential for baked-sauce chicken. Cornstarch contains zero gluten proteins and is pure starch; when it absorbs the egg and hits hot oil, it forms an incredibly tight, crisp, glass-like network. This shield prevents the chicken from drying out during its long braise in the oven while resisting turning soggy.

Heat the ¼ cup of oil in a large skillet over medium-high heat. Once shimmering, add the chicken pieces in a single layer (work in batches if needed to avoid crowding). Sear for only 1 to 2 minutes per side just until a pale golden, crispy outer crust forms. The center must remain raw. Transfer the par-seared chicken directly into your prepared 9×13-inch baking dish.

In a separate bowl, vigorously whisk together the granulated sugar, ketchup, vinegar, soy sauce, and garlic powder until the sugar grains are mostly suspended.

The Oven-Braise Glaze Infusion: Pour the raw sweet and sour liquid uniformly over the chicken pieces in the baking dish.

Bake uncovered at 325°F for exactly 1 hour. Crucial: Every 15 minutes, open the oven door and use a spoon to thoroughly stir and flip the chicken cubes inside the liquid. As the oven's dry heat evaporates the water content from the vinegar, the sugar molecules undergo rapid concentration. Stirring every 15 minutes redistributes this reduction, enveloping the crispy chicken shells in a thick, sticky, high-gloss lacquered glaze that locks onto the crumb.

Remove from the oven and let it rest for 5 minutes. The sauce will continue to tighten and thicken heavily as it drops slightly in temperature. Serve hot over a bed of steamed white rice.