Available as a kit, including white merino roving for making the thrums.

You can also download just the pattern here.

This technique for making extra warm garments has been around for ever and can be used for hats & socks as well.“Thrumming” places wisps of merino wool top (unspun but cleaned wool) into knit stitches periodically, leaving the tails of the wisps to the inside of the mitten. As you knit, the thrums build up on the inside of the mitten creating a thick and insulating layer of wool beneath the yarn.