Make tartar sauce in advance. Mix all the tartar sauce ingredients in a bowl. Set aside in the refrigerator.

In a small pot, heat soy sauce, rice vinegar, and sugar together until the sugar has completely dissolved, forming a vinegar sauce. Pour out in a shallow dish (large enough for dipping the chicken later). Set the vinegar sauce aside.

Butterfly chicken breasts, and pound with a meat mallet to flatten and thin to about 1/2" (1 cm). Sprinkle with salt and pepper. Coat chicken with flour, then dip in beaten egg.

Heat deep-frying oil to 350 F (180 C). Fry chicken until golden brown and cooked through.

Dip fried chicken immediately in the vinegar sauce, to cover both sides. Cut chicken into bite size pieces or slices. Serve with the tartar sauce.