I have no idea of what is this called, but I'm pretty sure Hata Hata is either the name of the circle or the artist. So I'd really appreciate if anyone could provide the title of this short, but cute doujinshi.
There's a preview of one I can't find: http://www.pixiv.net/member_illust.php?mode=medium&illust_id=37547314 If anyone could tell me where to find the full version of that one, I would appreciate it.