An article on About.com presents the rule:
In nearly all of Latin America, the c also has the "s" sound when it comes before an e or an i.
This is much like the situation in English, where what is basically the
same sound is heard in words such as "faces" and "fanciful." When c comes before other vowels, such as a or o, it retains the "k" sound.
But the "c" sounds like an "s" in cocinar so probably this is one of the many exceptions.
