In a medium bowl, whisk together the cornflour, flour, baking powder, and salt for the batter, then place the bowl in the fridge while you prep the rest. Keeping the batter cold makes a big difference in how crispy it turns out.
Lay the chicken on a cutting board, cover it with a piece of parchment paper or a freezer bag, and gently pound each piece until it’s about 0.7 cm (1/3 inch) thick. Sprinkle both sides with salt.
Spread the cornflour for dusting on a plate and lightly coat each piece of chicken on both sides, shaking off any extra. Set the chicken aside while you heat the oil.
Pour enough oil into a heavy-based pot to reach about 4 cm (1.6 inches) deep and heat it to 160°C (320°F). Make sure the oil is hot enough before frying, or the coating will soak up too much oil and lose its crunch.
Take the chilled batter from the fridge, add the soda water, and whisk just until combined. Don’t overmix it. A few small lumps are fine. The batter should be thin enough to coat the chicken smoothly.
Dip one piece of chicken into the batter, let the extra drip off, and carefully lower it into the hot oil. Fry each piece for about 3 minutes until it turns a light golden color. Work in batches so the oil doesn’t cool down too much.
Remove the fried chicken and set it on a paper towel-lined tray. Let it cool for 20 minutes. This short rest helps the coating firm up before the final fry.
While the chicken cools, make the lemon sauce. In a small saucepan, whisk together the cornflour and a splash of chicken stock until smooth. Add the rest of the chicken stock, lemon juice, sugar, garlic, ginger, and the rice vinegar mixture. Whisk and bring it to a simmer over medium heat, stirring every so often until it thickens to a thin honey-like consistency. Keep it warm.
Heat the oil again, this time to 200°C (390°F). Fry the chicken pieces a second time for about 3 minutes, turning halfway through, until they’re deep golden and crisp. Transfer them to fresh paper towels to drain.
Slice each piece into 2 cm (3/4 inch) strips, arrange them on a serving plate, and pour the warm lemon sauce over the top. Add lemon slices and green onion if you like, then serve right away with rice or noodles.