Trying to choose a favorite pintxo from San Sebastián makes us throw up our hands and sing the end of the first verse of a particular 1965 Beatles song: ‶…in my life, I loved them all.″

That said, we’re haunted by a simple slider on the menu at A Fuego Negro (31 de Agosto; tel: 650 135 373, www.afuegonegro.com/). The current incarnation is listed as ‶MakCobe with txips,″ which is a smirking play on words in English, Spanish, and Euskara that you almost have to be there to appreciate. We do know that you do have to be there in person to appreciate the little burger. Sure, the meat is richly beefy and meltingly tender. That’s to be expected. What blew us away was the ketchup-flavored bun made on the premises.

We have tried to replicate it several times, working with base recipes for brioche buns, potato rolls, and conventional burger buns. All were flops. The necessary tang of ketchup seems to keep the yeast from working very well, and the flavor is never just right. (It is, however, a good excuse to make hamburgers on the grill.) It’s a poor substitute for a week of eating in San Sebastian, but we take what pleasures we can.

If any reader has a successful ketchup-flavored bun recipe, we’d love to try it. (Hint, hint….) Send it to our email on the menu.