The pattern is from knitty:
I used cotton-ease, because my LYS didn't carry Rowan at the time, and also ka-ching! I made the second size and it took exactly one skein of charcoal (seriously, there's something great about looking at your scraps and not feel guilty that you'll never plan to do anything with it) and a quarter of their light blue color.
It was a very easy pattern to follow. Check it out!