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 teh 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.