I always thought it was the same fabric as the suit but now that I'm examining it closer it's starting to look more and more like it's sculpted. I found this
article that talks about the stitches on the entire costume being cast rubber, but it makes no mention of the cowl. The reproduction was made from garment grade sheet rubber and it was glued instead of sewn. I assume the original was also sheet rubber but it doesn't really specify.
shot it looks like the part that goes over the bridge of her nose conforms very well to the side of her nose. I would think if it were fabric it'd make a more or less straight line from the bridge of her nose to her cheek bone, leaving a gap where it doesn't touch the skin. Would sheet rubber be able to conform to her face like that?
shot the edges of the cowl around the eyes look a little "thick" for lack of a better word. More like what the edge of a Halloween mask might look like versus a cloth cowl. Again, I suppose sheet rubber could also look like that but having never worked with it I can't be sure.
Hopefully some of you movie buffs out there will know what her original cowl was made of. If not, I may try emailing the original costume designer and the designer who did the reproduction. Who knows, I might just get an answer!