Working with one chicken breast fillet at a time, using a sharp knife slice through the middle of the chicken as evenly as possible so it open up like a book (make sure to NOT cut all the way through). This is called butterflying. Set aside and repeat with remaining chicken fillets.
Cover the chicken breast fillets with a layer of cling wrap and pound out with a meat mallet until they are even in thickness.
Dust each fillet with plain flour to coat, shaking off the excess, then dip into the egg wash to coat, then straight into the Crumb Mixture, turning on each side and pressing in with your fingertips to secure the coating. Transfer to a lined baking tray and repeat with remaining chicken. Cover and chill for at least 30 minutes or up to 2 hours ahead of time.