In a small saucepan, combine all the sweet and sour sauce ingredients. Bring to a boil then set aside.
Chop the chicken breasts into 1-inch cubes. The rounder the shape, the better.
In a medium bowl, whisk together the flour, cornstarch, sugar, baking powder, and salt. Whisk in the cold water until you have a smooth and thick batter. The consistency should be similar to pancake batter without the bubbles. Add extra water or flour as necessary.
Heat about 2 inches of oil to 365°F in a Dutch oven. Working one at a time, dip a piece of chicken in the batter, shake off excess, and lower into the hot oil. Fry 6-8 balls at a time for 6-7 minutes, flipping frequently so they puff up evenly, until golden brown and crispy. Transfer to a wire rack to drain excess oil.
Reheat the sweet and sour sauce then serve.
For extra-crispy chicken balls, you can double-fry them at 400°F for one minute.