FROM WICKTIONARY
Etymology
From the association of the form of a face with the form of a pancake.

Noun
Pfannkuchengesicht n

(derogative) a very flat and broad face
Die hat voll das Pfannkuchengesicht!
(derogative) a person with a very flat and broad face
He, Pfannkuchengesicht!

Not sure that there's a particular word for this, or if there is, I've never heard it.
A "pancake face" is something you make children to eat (pancake with fruit etc. on so it looks like a face).

If you want to be creative:
A face that looks like [whoever's face it is] walked head-on into a bricked wall/had a close encounter with a flat immobile surface/had a date with a clothes press/fought a losing battle against a demolition ball etc.

Depending on how "serious" the context is.

Otherwise "flat-faced"

@ sjm, #3: "creative" ??? - I daresay: 'destructive'! . . .

BTW, for me, the girl with a face resembling a 'Pfannkuchengesicht' most (and in the most positive way), was/is the lead-singer of the group 'Pussycat', the one which had a world hit with 'Mississippi' . . .

http://www.wwwords.co.uk/pdf/viewpdf.asp?j=ci...
Mother B related a story of when her daughter came home from school and said that a friend told her that she had a pancake face.

http://justjared.buzznet.com/2007/06/25/camer...
She must be so proud to have fans like you! Good for her! Score, Cam!! You rule (in the opinion of your fan here). Keep working on that butt to keep your fans happy. As for your question, her butt doesn?t look special to me. But it does go beautifully with her pancake face.

http://www.urbandictionary.com/define.php?ter...
2.pancake face
having a flat face like a pancake.
also known as "2d face".

It's not just for breakfast anymore. :-)