Lightly pound the chicken breast ½ inch thick and season with salt. Put the panko, flour, and eggs in 3 separate pie plates, and season each one lightly with salt. Dredge one piece of chicken in the flour, tapping off the excess. Dip in the egg, allowing the excess to drip off, then coast the chicken with the panko, pressing to hep it adhere. Transfer the chicken to a wax paper-lined baking sheet. Repeat with the remaining chicken.
Heat oil in a large cast-iron skillet over medium-high heat until shimmering. Add the chekcn and fry, turning once, until golden and crispy on both sides, about 3 minutes per side. Drain on paper twoels. Serve the chicken with the katsu sauce.
To make, combine all ingredients in a saucepan and bring to a boil. Reduce the heat to medium low and continue cooking until the mixture reduces by half. Remove from the heat and chill.