In a medium saucepan, combine brown sugar, ketchup, lemon juice, vinegar, and water. Stir in the cornstarch until smooth. Place the saucepan over medium heat and bring the mixture to a boil, stirring constantly for about 5 minutes until thickened. Once the sauce has reached a glossy consistency, remove it from heat and let it cool slightly while you make the chicken balls.

In a large mixing bowl, whisk together all-purpose flour, cornstarch, sugar, baking powder, and coarse kosher salt. Gradually pour in cold water, whisking continuously until a thick batter forms that can coat the chicken pieces evenly.

Heat a deep skillet or heavy-bottomed pot over medium-high heat, adding enough neutral oil to reach a depth of about 2 inches. Once the oil reaches 375°F (190°C), dip each chunk of chicken into the batter, allowing excess batter to drip off, before carefully placing them into the hot oil. Fry for 3-4 minutes per side, or until the chicken balls are a deep golden brown and fully cooked through.

Using a slotted spoon, remove the crispy chicken balls from the oil and drain them on a wire rack or paper towels to remove excess oil. Serve the hot Crispy Chinese Chicken Balls alongside the sweet and sour sauce for dipping.