
Panini is already plural, so it certainly shouldn't have an apostrophe and then an "s" after it.
The subject of this entry is in honor of 95-year-old Abe, who lives with my grandfather at an assisted-living residence. He once complained to me that their dining hall uses fancy foreign words that he doesn't understand. "Do you know what a panini is?" he asked me. "It's just a sandwich with stripes!" I felt bad that he was so annoyed, but it was adorable.