The RS guide to Barcelona recommends a dessert stop at Granja M. Viader, tucked away behind an interesting church on the west side of the ramblas. It's a family shop in business since about 1870, now under management of the 5th or 6th generation.
http://granjaviader.cat.mialias.net/?page_id=5
I was chatting with them about my taste for dark chocolate, and they advised me to give Simon Coll a try -- a xocolater even a generation or two older than them. They have an original and a second factory about 40km inland, and offer tours and samples, but even better is to do a combination visit with one or more of the surrounding cava vineyards.
http://www.simoncoll.com/ca/general:Cos/visita#62
Accesible via line 4 of the local train system -- I think this is going on my to-do list for my next trip to Barcelona!
I also have in mind (speculative, I know) that the history of chocolate and candy-making factories farther north in Europe tends to involve Jewish families, and that dessert cafes are a way of melding the trade enterprise of Jewish businessmen with the farm owning and work of dairy producers, so I'm intrigued to try and learn more about the Simon Coll family background.