I tried to find an out of the way, unheralded, previously-unknown-to-all-but-portenos empanada place to rave about - I really did. But I kept getting reheated, precooked empanadas (even when I opted for table service at Guerrin (Corrientes 1368). And so, my best empanada in Argentina was at the much-heralded La Cupertina. The front window is covered in articles trumpeting how awesome it is. You have to ring a doorbell to get in. And it really is good.
Everything was delicious (I appreciated the texture of the meat in the beef empanada, which is finely chopped, not ground) but my award for best empanada goes to the round, double crusted one stuffed with bitter greens and chopped hard-boiled egg, dusted with a teeny bit of sugar.
La Cupertina also takes the prize for my favorite dessert in BsAs - in general, I found desserts to be sweeter than I like (I didn't find a single alfajore I could eat unaccompanied by black coffee, and even the lovely dark chocolates at Tikal Chocolates in Palermo were too sweet for me). La Cupertina's Torta with Dulce de Leche Mousse does have a sugary meringue cap, but underneath are two layers of rich, not-too sweet butter cake sandwiching a thick, airy caramel mousse.
Cabrera 5296 (Palermo)