The fabric is a nice medium weight cotton/ lycra (on sale right now, and no, I'm not an affiliate!) from Girl Charlee. I originally bought it to make a bodysuit, and it would be a good weight for that too, and I may have enough leftover for a short-sleeved version.
I've been on a knitting kick lately (easier to do while vegging on the couch in the winter!), so you can spot one of my creations in the photos here. I have to block a second hat, but then I'll give the details of both. :)