I suppose the best sweets for souvenir is Shiroi koibito (White Lover)!
http://best.ciao.jp/ohkashi/database/database.cgi?cmd=dp&num=1&dp=
This is the most popular souvenir from Hokkaido which has been popular for over 30 years since its birth. It’s a Langue-de-chat-type sweet, whose cookies are baked to golden perfection and sandwiching white chocolate. The crunchiness which is a key to this cookie is produced under strict quality control in the factory performing thorough hygienic management.
Please dip this Shiroi koibito in cold milk! It's yummy!