Aroma Cafes tend to have plugs. But don't look for WiFi at the Aroma on Calle Florida at Plaza Gen. San Martin... because you're not going to find it.
The one at Callao y Corrientes, Quintana @ Recoleta Cemetery, & maybe the one at Santa Fe y Uruguay are better bets. You'll find the plugs in what they call "El Living". (More of a living room/ lounge than a typical coffee shop setup.) There are other Aromas as well on Santa Fe, so keep your head up and you'll probably find some power outlets to go with your WiFi y crapo cafe.