Her questions reminded me that chocolate and vanilla are the standard, "basic" flavors you find in the U.S. A small ice cream shop might only have those two options, and you'd expect a larger shop to offer chocolate and vanilla (by default) plus other options.
More deeply, my friend's questions made me reflect on what's "standard" or "plain"--and how that's culturally-bounded.
In the U.S., we refer to something as "plain vanilla" to indicate the option that has little embellishment or elaboration. For example, a "plain vanilla" outfit, car, or house is the simplest version offered. As an expression, "plain vanilla" holds a lot of cultural meaning.
Incidentally, in Italy, you're just as likely to encounter a small gelato shop that offers sour cherry, hazelnut, or nougat as you are to see "plain" chocolate. Vanilla, on the other hand, is pretty exotic. The standard, "unmarked" base flavor is "gelato alla crema," made with milk, sugar, and egg. Vanilla, as an aroma, is as noteworthy in Italy as rosewater or orange blossom would be in the U.S.
(Needless to say, my friend and I both enjoyed all the flavors we tried!)
What expressions do you use that reveal your cultural frame or bias? How might these expressions translate across cultures? What do they reveal about your expectations of "how things should always be?"