There are many answers to this question, ideas of using frozen pieces or adding enzymes to hollowed out chocolate shells, these are all bogus, here's the real method. They begin with a mold of a Caramilk bar which they place upside down, they pour some chocolate into it, enough to create a layer but still have a little pocket. The chocolate cools, then they pour caramel into the mold, they cool the caramel, and then pour the rest of the chocolate on top. Quite simple and the only real trick to it is that the bars are made upside down. Check the link out for a more complete explanation of the process, as well as some of the crazier ideas at the bottom.
Or you can buy your own chocolate here:
Ghirardelli Chocolate
Here's a link that will help you understand the Caramilk Secret.
http://members.shaw.ca/b.bogdan/caramilk/cadbury.htm
