In piano cucina the main noun is piano (meaning “surface”, “top”), while cucina (“kitchen”) is a kind of attribute.
A more traditional form might be piano da cucina or something like that, but in Italian it is not infrequent to find phrases of the form noun+noun, written together or apart. Consider pescespada (“swordfish”), scuola guida (“driving school”), busta paga (“pay packet”), carrozza ristorante (“dining car”), treno merci (“goods train”) and lots of other ones.
In all these cases the second noun modifies, makes more specific the first one, which is the one to be considered for the gender and number of the whole expression.