Creme brulee has hard top layer
I had this happen to me - it's because fat gathers on the top of the creme.

After heating the creme, for some reason, fat/oils gather at the top of the creme and when cooled become the hard layer you have described. It's just what normally happens to fat when you cool it down - it gets hard.

You can try stirring your creme before letting it cool down completely, unfortunately I do not know why this happens or how to fix it, otherwise.

EDIT:
Apparently, as @mathgenius pointed out, this happens if you use vegetable oil based cream. He reported that when using animal far cream the problem was fixed.