Thanks for the update
A small correction, what Koshiba is eating is not a "Pola" but a "Porra", I was puzzled as to what it was seeing this is Barcelona and was scratching my head hard thinking about what this "Pola" is. Understandably, in Japanese it was probably written as 「ポラス」or 「ポーラス」, so unless you know the actual name, writing it properly in romaji is unlikely.
A small rant as well: the heck is the author doing... there are no Porras in the Barcelona region, you can look hard and you won't find any around Catalonia, and very likely around a good chunk of Spain. Porras are typical of the Madrid area, finding them outside of that is unlikely, unless you can find a specialized bar or coffee shop that serves them. You might, MAYBE find churros at a truck stall (a real rarity) during some town celebrating a "festival", and while I heard these stalls used to serve porras a long time ago, they certainly have not in the past 20+ years. In fact, they don't even serve hot chocolate to go with the churros anymore (yes, churros and porras are supposed to be eaten by dipping them in hot chocolate).
Had I been the author, I might have opted for a specialty of Catalonia, since Barcelona itself, nor the entire region of Barcelona, has any specialties at all... a secallona or a bocata might have been a good option I think, it fits the image the author wanted to convey, eating on the street, which are basically a thinner, tenderer version of spanish chorizo and a baguette sandwich... actually, there is one specialty in Barcelona city, and that's the bikini, which is essentially a grilled ham and cheese sandwich.