Begin by cutting the chicken breasts into ½-inch thick strips and set aside.

In a large bowl, whisk together cornstarch, all-purpose flour, baking powder, sugar, and salt. Gradually add cold water until smooth and slightly thick.

Heat about 2 inches of oil in a heavy pot to 350°F (175°C). Use a thermometer for accuracy.

Dip each chicken strip into the batter, ensuring a generous coating while letting excess drip off.

Carefully lower the battered chicken strips into the hot oil in batches of 5-7, frying for 6-7 minutes until golden brown.

Transfer cooked chicken fingers to a wire rack over a baking sheet to drain excess oil.

In a saucepan, combine ketchup, brown sugar, white vinegar, cornstarch, salt, and lychee or pineapple juice. Boil over medium-high heat while stirring until thickened.

Serve the crispy chicken fingers hot with the sweet and sour sauce on the side.