I thought that the apostrophe was not necessary in the case of Cosa. Cosa hai avuto would be correct ?
it is correct, but cos'hai is perfect. Generally, italians avoid to leave the vowel before the H if it's possible to insert the apostrophe
I would say yes. For example if the breakfast referred to is not the one from this morning, but maybe the breakfast you usually had on your last trip.
Is using "essere and "avere" interchangeable in past perfect? for instance, can I also say " Cosa sei avuto a colazione"? thanks
No! "Cosa sei avuto" is really wrong! The verb is coniugate in "passato prossimo" in this sentence and it is always formed by verb "avere" + past participle
This Italian sentence is very poor! it follows and repeats the English construction. None in Italy would say that! An Italian could say: "Con cosa hai fatto colazione?" or "che hai mangiato a colazione?"