Put the chicken in a bowl, and toss with soy sauce. Place in the refrigerator to marinate for 30 minutes.
To make the sauce, in a small saucepan on medium heat, whisk together the lemon juice, lemon zest, sugar, water and cornstarch. Bring to a boil, whisking constantly until thickened, then keep warm.
Meanwhile, heat the oil in a large skillet. Whisk together the egg yolk and cornstarch in a small bowl. Toss the chicken in this batter and fry for 3-4 minutes on each side, until golden and cooked through.
Transfer to a plate and spoon the warm sauce over the chicken.