Ooo, is that cheesecake?
A running gag about a film character's facial expression - in this case, a look of anything from dull to alert to delighted interest, in something out of view of the capper. Possibly a tempting treat. (Possibly just a blank look too. But that's an existential sort of question.)
First 'historical' Babylon 5 usage as archived on this page. (Found frequently among the various captions on successive pages of the same capper site).
[Example from 'Akce Bororo', a 70s Czech movie.]