Would you be guilty of a malapropism if you said that you went out with your friends for piazza rather than pizza?

Yes, saying you went out for "piazza" instead of "Pizza" would be an example of a malapropism. A malapropism occurs when a Word is mistakenly used in place of a similar-sounding one, often with humorous effect. In this case, "piazza" refers to a public square or marketplace, which is quite different from pizza, a type of food.