In a large mixing bowl, combine 1 cup of all-purpose flour, 1 cup of cornstarch, 2 teaspoons of baking powder, 1 tablespoon of granulated sugar, and 1 teaspoon of salt. Whisk until blended, then gradually pour in 1½ cups of ice-cold water, mixing until the batter is smooth and thickened.

Thoroughly pat dry your bite-sized chicken pieces. Dip each piece into the prepared batter, ensuring it's fully coated.

In a deep fryer or heavy pot, heat vegetable oil to 350°F (175°C). Carefully place battered chicken pieces into the hot oil in batches, fry for about 3–4 minutes per side until golden brown.

In a medium saucepan over medium heat, combine ½ cup of brown sugar, 3 tablespoons of ketchup, 2 tablespoons of vinegar, 1 tablespoon of fresh lemon juice, and ½ cup of water. Whisk until the sugar dissolves, then gradually stir in 1½ teaspoons of cornstarch mixed with a splash of water, heating until the sauce bubbles and thickens.

Arrange the crispy chicken balls on a serving platter and serve them hot alongside your sweet and sour sauce.