Chill the club soda and place a mixing bowl in the freezer for 5 minutes.
Slice chicken into thin strips. Pat dry, then season with salt, pepper, and garlic powder.
Heat oil in a deep pot to 350°F over medium heat. Set a rack over a sheet pan.
Whisk soy sauce, mirin, rice vinegar, sugar, and ginger in a small bowl. Set aside.
Whisk flour, cornstarch, and baking powder in the chilled bowl.
Beat the egg lightly, then stir it into the club soda. Pour into dry ingredients and mix briefly. Leave lumpy.
Dust chicken lightly with the extra flour, shaking off excess.
Dip floured chicken into the cold batter, letting excess drip back into the bowl.
Fry chicken in batches for 3–4 minutes until pale golden and cooked through. Avoid crowding.
Drain on the rack and season with a pinch of salt. Serve hot with the dipping sauce.