Sant Antoni is where it's at right now, so many of the loveliest places can be found there. The cosy La Marmota is located on Comte Borrell, close by the Sant Antoni market. Here you can find the two owners, coincidentally both called Maria: one from Andalusia and one from Finland.
Outside there is a small, very inviting veranda-like terrace, and once you step inside you'll find a homey little cafe. This is the kind of place you feel you can stay for hours, chatting away with your friends or maybe working on your laptop (they have wifi).
The service is tremendously friendly, and the menu is great. You'll find all the comfort food you need here, maybe it's a carrot cake you're craving, a good smoothie or a nutella croissant. They have savoury items too, sandwiches and salads to name a few. The Marias put their respective signature foods in play too: I can really recommend you'll try the typical Finnish dark bread. It's soft and sweet and goes really great with smoked salmon and cream cheese.
I can't say I know any café in Barcelona with such a lovely, warm atmosphere, I really recommend that you'll try it out. You will not want to leave for hours.