Womens Butter Rib Racerback Tank XS - Blizzard HTHR. The Butter Rib Racerback Tank is a super-soft, super-comfortable and super-breathable top for your summer camping trips, warm-weather travel, or kicking back after a long day on the trail. The Butter Rib Tank's slim-fit racerback design makes for easy active crossovers, and the incredibly soft 100% merino wool rib fabric brings added coziness to your down time. Wear it with your Butter Rib Hot Pants for maximum coziness.