Chicken breasts are lean, affordable, and oh-so-versatile, but their uneven shape presents an eternal problem for cooks. The narrow side gets overdone while the thick side finishes cooking.
Butterflying –- that is, splitting the chicken breast horizontally and opening it like a book –- allows you to then easily pound the meat to an even thickness. (Prepared this way, the split and pounded chicken breasts are called paillards.) Alternatively, you can stuff that butterflied chicken breast with something delicious and close it back up for cooking.
Place one hand on the chicken, curving your fingers up slightly for safety. Slice meat horizontally, cutting almost to the other side. Open it like a book.
Place meat inside a plastic bag or between two sheets of plastic wrap. Working from the center out, pound with the smooth side of a mallet until meat is 1/8 to 1/4 inch thick.