Place the sliced chicken in a mixing bowl and toss with soy sauce, salt, and pepper. Set aside to marinate briefly while you make the batter.
In a separate bowl, whisk together the all-purpose flour, cornstarch, and baking soda until evenly combined.
Make a well in the dry ingredients and add the whisked egg and cold soda water. Gently mix until just combined; the batter should be slightly lumpy and airy.
Heat about 1 cup vegetable oil in a deep pot or saucepan over medium heat until it reaches about 350°F (hot but not smoking).
Working quickly, dip each piece of marinated chicken into the batter, coating it completely, and shake off any excess.
Fry several pieces at a time in the hot oil for 2–3 minutes until golden and cooked through. Do not overcrowd the pot.
Remove the cooked chicken with tongs and drain on paper towels. Repeat frying with remaining pieces.
