On my last night in Buenos Aires, went to this super cool bar called Floreria Atlantico. You have to walk through a small flower shop/wine store and go down these rickety spiral stairs to get to the bar. Loved it!
Among the warehouse-feeling racks of bottles, they had a great wine list and awesome tapas (we had BBQ octopus and chard fritters with mint sauce) but I wound up getting cocktails. Mine was called the Transatlantico Fizz and had whiskey, egg whites and yerba mate in it among a few other ingredients (see menu below). So good and such a cool find on my last night.