Prepare the Sweet and Sour Sauce: In a small saucepan, combine water, ketchup, brown sugar, vinegar, and lemon juice. Whisk in cornstarch until fully dissolved. Bring to a boil over medium heat, stirring. Once thickened, reduce heat and simmer for 2-3 minutes, then remove from heat and cool.

Preheat the Oil: In a deep pot, pour in about 1 inch of vegetable oil and heat to 350°F. Monitor closely for perfect crispy texture.

Mix the Batter: In a mixing bowl, combine flour, cornstarch, sugar, baking powder, and salt. Gradually add cold water, stirring until smooth and thick but pourable.

Prepare the Chicken: Slice chicken breasts into uniform strips about 1 inch wide. Pat dry with paper towels. Dip each strip into the batter, covering completely.

Fry the Chicken Fingers: Lower battered chicken strips into hot oil one at a time. Fry for 2-3 minutes on one side until golden brown, then flip and cook for another 2-3 minutes.

Drain and Serve: Use a slotted spoon to transfer chicken fingers to a plate lined with paper towels to drain excess oil. Serve hot with sweet and sour sauce.