Cut the chicken breasts into strips and place them in a bowl. Add 2 1/2 tablespoons soy sauce, 1/2 teaspoon salt, and 1/2 teaspoon black pepper. Toss to coat evenly and set aside to marinate while you prepare the batter.
In a separate bowl, sift (or whisk) together 3/4 cup all-purpose flour, 5 tablespoons cornstarch, and 1/4 teaspoon baking soda.
Add 1 large lightly beaten egg and 1 cup cold sparkling water to the dry mixture. Stir gently with a fork or spatula until just combined; the batter should remain slightly lumpy. Do not overmix.
Pour vegetable oil into a deep pan or pot and heat it to 350°F. Use a thermometer if available. Keep the oil at this temperature while frying.
Lightly shake excess marinade off the chicken strips. Working in small batches so the oil temperature stays near 350°F, dip each piece into the batter, letting excess batter drip back into the bowl.
Carefully add battered chicken to the hot oil and fry 2–3 minutes per side, or until the batter is crisp and the chicken is cooked through. Do not overcrowd the pan; adjust batch size as needed to maintain temperature.
Remove the fried chicken with a slotted spoon and drain on paper towels. Repeat with remaining chicken.
Serve immediately with your preferred dipping sauce, if desired.