The pictures show the hats made in pure MILK fiber by Viking of Norway, colorway 969 (big one) and 970 (small one). One ball of 180 yards/50 grams should be enough for at least two hats. However, any kind yarn will do as long as the gauge is the same.

Errata:
Small size - k2,M1L,k22,k2tog,k2
Big size - k2,M1L,k28,k2tog,k2

The pdf. on my site has been updated, and the pattern is now correct.