It's a dynamic phrase that could be used to say someone's
attractive, or if food looks good. It's antonym is "cook it down" which could mean that someone's nasty, shouldn't
sport that type of clothing, or if the food isn't as good as
you thought.
(
hot chick jogs by) "Mmm, cook it up!"
"What's for lunch today?"
"
Chicken fajitas."
"Mmm, cook it up!"
(someone girl's pasty ass crack
hanging out) "Uhh, cook it down! Cook it way down!!"