Chunky White-Chocolate Chip Cookies

This variation tastes distinctively different from regular chocolate chip cookies, and the chunks of melted white chocolate on top give the cookies lots of visual appeal. Every once in a while, one of my students will produce a batch of white chocolate chip cookies in which the chunks of white chocolate have vanished. These always appear normal going into the oven, but they come out extremely flat with no visible chunks of chocolate left. What invariably causes this mysterious disappearance