There is also the related phenomenon where we take a foreign plural and use it as a singular. Then we sometimes add an s to that for the/an English plural. I lived in Italy for several decades and whenever I returned to the U.S. it was always jarring to hear a café employee ask "Would you like another biscotti?" or ring me up with "So that's one espresso and two biscottis."
M-W defines the singular:
A crisp cookie or biscuit of Italian origin that is flavored usually with anise and filberts or
almonds —usually used in plural m-w
I assume usually used in the plural means biscotti is OK as a singular.
The OED defines the singular headword biscotti and gives two plurals: "Plural unchanged, biscottis." It puts biscotto under Forms
For what's cooking, see this rather unambiguous Ngram
Collins Cobuild takes another approach. They define the plural noun but keep shtum about biscottis:
biscotti PLURAL NOUN
Word forms: singular biscotto or biscotti
Hard, plain, bar-shaped cookies containing almonds or hazelnuts Collins
Isn't that like defining dogs and telling you the singular form is dog? However, Collins seems to define its cookies, like macaroon (countable noun), in the plural. I guess you can't eat just one.
Summing up, the Italian biscotto/biscotti has become:
Singular | Plural
Something similar happened with the Italian singular panino:
variants: or less commonly panino
plural panini or paninis
NOTE: Panini is a plural form in Italian but is commonly used as both
a singular and a plural in English. Its use as a singular has given
rise the variant plural form paninis. The use of panino as a singular
form in English has become relatively uncommon as panini has become
established as the usual singular. m-w
Sticking with Italy, there they call each noodle a lasagna and the dish lasagne (since they do serve you more than one noodle). We usually call both the pasta and the dish
1 or less commonly lasagne : pasta in the form of broad often ruffled ribbons
2 a baked dish containing layers of boiled lasagna, and usually cheese, a seasoned sauce of tomatoes, and meat or vegetables m-w
The Italian brand, Barilla, has lasagne on their box of dry noodles. The Muller's brand seems to have an existential crisis. I see pictures with lasagna on some boxes and lasagne on others. In one picture, the instructions on the back of box for making lasanga include "16 oz. Muller's lasagne, cooked 7 minutes, drained"