Line a plate with paper towels. Set aside.
In a medium bowl, whisk the tonkatsu sauce ingredients together. Set aside.
Slice each chicken breast lengthwise into two pieces. Season both sides with salt and pepper.
Create a breading station. Add the flour, eggs, and Panko each to separate shallow bowls.
Dredge the chicken in the flour, shaking off excess; then the eggs, letting any excess drip off; and finally, the Panko.
Fill a large cast iron skillet with ¼-inch oil. Heat over medium until it reaches 350°F.
Place the chicken, 2 pieces at a time, into the oil, cooking on both sides until golden and 165°F internally, about 2 minutes per side.
Transfer to the prepared plate and repeat with remaining chicken breasts.
Serve hot with tonkatsu sauce.
