Butter candy is a candy made from butter and it is a typical Hokkaido souvenir. It was first produced in Yakumo town and still as the local sweet of Yakumo but butter candies are sold by other companies out of Yakumo too.
Butter candy was the popular for tourists visiting Hokkaido, but the popularity has been decreased after many other confections. Therefore it is getting hard to find at stores at airports and stations.
On the other hand, butter candy is still main product of Tobetsu Trappist monastery which is located outside of Hakodate and popular tourist destination for tourists visiting Hakodate.
Anyway, butter candy is very worth to purchase and if you visit Hokkaido, go ahead to grab the bag and bring it to the cashier.