In a large mixing bowl, combine all-purpose flour, cornstarch, baking powder, granulated sugar, and salt. Gradually whisk in ice-cold water until the batter is smooth and free of lumps.

Pat the chicken dry with paper towels, cut into bite-sized pieces, and dip each piece into the prepared batter, letting excess drip off.

Heat vegetable oil to 350°F (175°C) and fry the battered chicken pieces in batches for about 3-4 minutes until golden brown. Drain on a wire rack.

In a saucepan, combine brown sugar, ketchup, vinegar, lemon juice, and water. Whisk in cornstarch and additional salt, heat over medium until thickened.

Serve the crispy chicken balls on a platter and pour the sweet and sour sauce into a bowl for dipping.