Hi, if I said atah chonantanu before tzait hacochavim, do I need to say Baruch hamavdil after tzait, before doing melacha, and do I need to make havdala al hakos at some point after tzait?
A person should not daven before tzait hacochavim on motzei Shabbos before the zman. Your question however was if it was already done. If he did daven, he was yotza havdalah, and he doesn’t have to say boruch hamavdil, because he already did the mitzvah of havdalah, which is to declare the shabbos is special.
He does have to make havdalah on a cup of wine like everyone else, and he may not eat until he does so.
He also has to remember to say kriyas shema again.
O:CH 293-3, M:B ibid 10, Rambam Hilchos Shabbos29-11.