These taste just like the ones you get from Chinese takeout. Most recipes use self-rising flour in the batter, but in my experiments, baking powder worked just as good.
Cut the chicken breasts into ½-inch thick strips and set aside.
In a medium bowl, whisk together the cornstarch, flour, baking powder, sugar, and salt. Whisk in the cold water until you have a smooth batter. The consistency should be thick enough to coat a piece of chicken but slide off of it slowly. Add extra cornstarch or flour as needed.
Heat about 2 inches of oil to 350°F in a large pot. Working one at a time, dip a piece of chicken in the wet batter, shake off excess batter, then lower it into the hot oil. Fry about 5-7 pieces at a time for 6-7 minutes until golden brown and crispy. Transfer to a wire rack to drain excess oil.
For the sweet and sour sauce, combine all the ingredients and bring to a boil over high heat. Enjoy!