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?"
"Mmm, cook it up!"
(someone girl's pasty ass crack hanging out) "Uhh, cook it down! Cook it way down!!"
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