The easiest way to break up peppermint bark is to use a knife.  Once the chocolate is completely set, use a small paring knife and poke to tip into the chocolate, causing it to break.  Continue doing this over the top of the chocolate until it’s into small to medium-sized pieces. Dark chocolate bark is likely to be of various shapes and sizes since I find it quite difficult to cut into uniform pieces like you would a tray of brownies.