On a large cutting board, lay your chicken breasts flat and butterfly each breast by equally slicing it halfway but not all the way through. You should be able to open the breast into two adjoined "wings".
Lay a piece of plastic wrap to cover the butterflied chicken breast to prevent the juices from splashing everywhere (optional). Using the back and side of your knife or a meat mallet, evenly tenderize the chicken breasts into thin slices, about ½-inch thick. Remove the plastic wrap.
Evenly season each chicken breast on both sides with salt and black pepper.
Place flour into a large shallow bowl. In the second bowl, beat your eggs. In the last bowl, evenly spread-out panko breadcrumbs. Evenly dredge each seasoned breast in all-purpose flour, then beaten egg and lastly in the panko breadcrumbs ensuring no bald spots.
In a large heavy-bottomed pan, heat vegetable oil on medium heat, around 350 degrees F with a digital instant read thermometer or insert a wooden utensil into the oil and look for bubbles.
Fry each breast one at a time until golden brown, about 2-3 minutes per side. Avoid moving the chicken around in the oil to evenly cook it. Cooked chicken has an internal temperature of 165 degrees F on an instant-read thermometer or until juices run clear.
Transfer fried chicken to a wire rack set over a baking sheet or paper towel lined plate to remove excess oil. Allow this to cool a bit for a few minutes.
In a small bowl, combine katsu sauce ingredients as listed. Serve and enjoy with chicken katsu!