Cut the chicken breast into bite-sized pieces — roughly 4–5cm. Don't cut too small or it will dry out.
Mix the sake, soy sauce, sesame oil, ginger and garlic in a bowl. Add the chicken, turn to coat, and marinate for at least 10 minutes.
Mix the cold water, egg, flour, and potato starch together until just combined. A few lumps are fine.
Heat the oil to 170–180°C. Dip each piece of marinated chicken into the batter and lower gently into the oil. Fry in batches for 3–4 minutes until light golden and cooked through.
Drain on a wire rack or paper towels. Serve immediately with shredded cabbage and a dipping sauce on the side.