Butterfly cut 450 g chicken breast so that the thickness is about 1.5-2cm (3/4 inch) all over.
Cut into large pieces, then pierce each piece all over with a fork. Pat dry with kitchen paper, then sprinkle with a pinch of salt and pepper on both sides and place them in a bowl.
Add 3 tbsp cake flour and 1 tbsp cornstarch to the bowl. Toss until evenly coated.
Crack 2 eggs into a separate bowl and whisk until the yolks and whites are combined. Shake the excess flour off of each piece of chicken before placing them in in the bowl with the egg.
Submerge the chicken pieces in the egg mixture, then cover and place in the fridge while you prepare the other components.
Tartar Sauce
Finely dice 1 small pickled gherkin, ¼ onion and 1 hard boiled egg.
Place them in a mixing bowl and add 4 tbsp Japanese mayonnaise, ½ tbsp lemon juice, 2 tsp whole milk, 1 tsp sugar, ½ tsp ketchup, 1 pinch salt and pepper and a pinch of parsley. Mix until everything is combined.
Cover and refrigerate until serving.
Nanbanzu
Take a small sauce pan and add 3 tbsp Japanese soy sauce (koikuchi shoyu). 3 tbsp rice vinegar, 2 tbsp sugar, 1 tbsp mirin, and 1 dried red chili pepper split lengthways and deseeded. Heat on medium and stir to dissolve the sugar, boil for one minute then take it off the heat and leave to cool to room temperature.
Chicken Nanban
Preheat a deep pot of cooking oil to 170 °C (338 °F). Once hot, hold up each piece of chicken to drain off excess egg and place them in the oil with the skin side facing down. Fry for 3 minutes.
Transfer to a wire rack and rest for 3 minutes. Increase the oil temperature to 180 °C (356 °F).
Add the chicken pieces back in and fry once more, this time for 2 minutes.
Rest on a wire rack for 15-30 seconds, then roll each piece in the nanban sauce for 10-20 seconds. Rest each piece on a wire rack while you coat the others.
Transfer to a serving plate and spoon the chilled tartar sauce over the top. Sprinkle with extra parsley for color. Serve with rice, shredded cabbage or your choice of sides. Enjoy!
Notes
Rinse minced onion under cold water for 30 seconds, then squeeze in kitchen towel to remove harsh compounds.You can replace half the milk and sugar with pickle juice for extra complexity.For traditional tender texture, use pure flour coating. For extra crunch (non-traditional), dust egg-coated chicken with cornstarch before frying.Dip hot fried chicken immediately into room-temperature nanban sauce. The temperature contrast forces liquid deep into the egg coating's air pockets for maximum flavor absorption.Storage: Store sauced chicken in airtight container for 1-2 days (coating will soften). Keep tartar sauce separate in its own container for up to 2 days.Meal Prep: Prepare components up to 24 hours ahead. Tartar sauce improves overnight, nanban sauce keeps refrigerated for 1 week, and chicken can be seasoned and butterflied in advance.Serving Ideas: Freshly cooked Japanese rice, Traditional miso soup, Cucumber Sunomono Salad, Spinach Ohitashi, Japanese Potato Salad, Hiyayakko (Cold Tofu).