Start by cutting your chicken breasts into bite-sized pieces. In a bowl, combine the soy sauce, sesame oil, garlic powder, and white pepper. Toss the chicken in the mixture and let it marinate for 15–20 minutes while you prepare the batter. This gives the meat more flavor and helps it stay juicy inside the crispy shell.

In a separate mixing bowl, whisk together the flour, cornstarch, baking powder, and salt. Add the beaten egg and cold sparkling water, mixing just until combined. The key to a great tempura batter is not overmixing — some lumps are okay. Keep the batter cold by placing the bowl over another bowl filled with ice or keeping it in the fridge until ready to use.

Fill a deep pot with 2–3 inches of vegetable oil and heat it to 350°F (175°C). Use a thermometer for accuracy — temperature control is crucial to getting that perfect golden crunch without greasiness.

Lightly dust the marinated chicken pieces with cornstarch — this helps the batter adhere better. Dip each piece into the tempura batter, let the excess drip off, then carefully lower it into the hot oil. Fry in batches, being careful not to overcrowd the pot. Each batch should take 4–5 minutes, or until the chicken is golden and fully cooked (internal temp of 165°F).

Transfer the fried chicken balls to a paper towel-lined plate or cooling rack to drain.

While the chicken rests, combine ketchup, vinegar, sugar, soy sauce, and garlic powder in a small saucepan. Bring to a simmer over medium heat. Mix the cornstarch slurry and stir it into the sauce to thicken. Simmer for another minute or two until glossy. Taste and adjust balance — add more vinegar for tang or sugar for sweetness if needed.

Serve the hot chicken balls with a generous drizzle of sweet and sour sauce, or offer it on the side for dipping. For an extra takeout-style feel, garnish with chopped green onions or sesame seeds.