Well based on this site (linked) they are double knit and heavily felted. If you join Ravelry you can get the pattern there for $4. This is the only one I could find.
Scroll down at this link for a photo of what the pattern hat looks like.
Here's a "fake" double knit that is not felted. If you just knit it longer before decreasing you'll get that extra length. Obviously not authentic though.