東方氷精姉 (とうほうひょうしょうし) Touhou Hyoushou-shi "Touhou Adult Ice Fairy"
Originally an animated doujin series created by Yurume Atsushi in 2009 featuring an adult Cirno, Atsushi continues to release new content on PIXIV.

Atsushi refers to adult Cirno as "Long Cirno" (she has long hair).

There are several sub-stories within the Touhou Adult Ice Fairy universe:
Minimum Max
Border of Cirno
Border of Scarlet
Triple Cirno
Cirno Cross

Original Niconico Video: Episode 1 of "Touhou Adult Ice Fairy"
https://www.nicovideo.jp/watch/sm6735453