Complex sentence: Foam cakes are cakes with very little (if any) fatty material such as butter, oil or shortening, and which are leavened primarily by the air that is beaten into the egg whites that they contain.
Split into simpler sentences: Foam cakes are cakes with very little (if any) fatty material such as butter, oil or shortening. They are leavened primarily by the air that is beaten into the egg whites that they contain.