Good stuff here:
http://www.deepsouthdish.com/2008/10/chocolate-toffee-bark.html#axzz2moWTygzC
I usually use semi-sweet chocolate chips rather than the broken chocolate bars. Also, just put the pan in the fridge to harden it up some and then just break into irregular pieces.
Decadent.....
Have checked a cooking chemistry book out of the digital library and will check back in with an answer if I find one. Apparently, you need a pastry brush and water to prevent crystals from seeding more crystals on the sides of the pan. Using the candy thermometer would have been better if you happen to know the temp optimal for making caramel or toffee.