In that case, I think they're called finger limes in English. I've seen them at Eataly before, and I'm sure you can mail order too. They are likely obnoxiously expensive.
You won't believe that it's butter.
I would guess that the citron caviar is probably not that necessary. There are some things in the book that seem like Mathieu Pacaud superfluously added to try to modernize/improve his father's...