Prick the chicken with a fork and cut into bite-sized pieces. Massage with the sake, soy sauce, ginger, and garlic and marinate in the fridge for at least 15 minutes.
Lightly whip the egg white until frothy, then mix in the cold water, followed by the flour and potato starch. Stir just until barely combined; a few lumps are fine.
Heat about 2 cm of oil to 170C. Dip the chicken in the batter and fry in batches, about 2 minutes, then turn and fry another 3-4 minutes until pale golden and cooked through.
Drain on a wire rack. Serve with shredded cabbage, cherry tomatoes, and ponzu for dipping.