Slice the chicken breast horizontally into two slices. Season with salt and black pepper. Leave to stand for 10 minutes. Mix all the ingredients for the Katsu Sauce in a bowl, stir to mix well.
Heat up a pot of cooking oil for deep-frying. Ideal temperature is about 350°F (180°C).
Dip each piece of the chicken into the beaten egg, then immediately coat it with the panko. Shake off the excess. Dip the chicken into the egg again, and then coat with the panko for the second time. Make sure that the chicken is evenly coated with a thick layer of panko. Repeat the same for the other piece of chicken.
Once the oil is fully heated, gently drop each piece of the chicken into the pot and deep fry until both sides turn golden brown. Dish out the chicken onto a plate lined with paper towels. Cut the chicken into strips, and serve immediately with steamed rice and katsu sauce.
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Notes
You can buy the bottled Katsu Sauce or Tonkatsu Sauce from Japanese or Asian food stores.