Strictly speaking, it is acceptable to prepare just a cooked food for eiruv tavshilin. The proper method, though, is to also prepare a baked item. If only a baked item was prepared, the eiruv is invalid. The cooked food is designed to allow cooking for Shabbos, while the baked food permits baking for Shabbos. The cooked food must be a food that is eaten with bread, as opposed to a food that is eaten independently. Examples include fish, meat and eggs. The baked food may be either bread, matzah or cake.
[שו"ע תקכז, ב ו־ד, ומשנ"ב ו, ז, יא ו־יב; ביאורים ומוספים דרשו, 15