How-to Butterfly a Chicken
It's easy to butterfly boneless skinless chicken breast halves. The result is chicken that will cook quickly and evenly because it is half of its original thickness. Using a sharp knife, make a lengthwise slit from the thickest side of a chicken breast to within 1/2 inch of the opposite side. Open chicken so it lies flat; cover with plastic wrap. Use the flat side of a meat mallet to flatten to the desired thickness.