Kakashi's face without the mask
It was originally shown in the book Naruto Exhibition Official Guestbook New Style: Scroll of Wind (or Fū no Sho for short), which was given to the guests who attended a Naruto exhibition held in Japan on April 25, 2015.

Fū no Sho:



It was then adapted into an anime episode (ep. 469), which you can watch here.

By the way, this is how he looks like: