First of all, we should mention that "food" in these sentences is used as a COUNT noun (i.e. it carries the meaning of "dishes", "kinds of food", if you prefer)
Having said that, the only real difference is that when you ask
what are your favorite italian foods?
You expect more than one item as an answer (e.g. spaghetti, pizza and tagliatelle)
When you ask
what is your favorite italian food?
You expect just one item (e.g. My favorite italian food is pizza)