Was Muriel Spark responsible for the phrase entering the lexicon, as I have seen claimed? Or did it already exist and her usage of it in The Prime of Miss Jean Brodie merely popularised its use?
I hope nobody really believes this.
According to the Trésor de la langue française, the use of crème to mean the best part of something is first attested in 1580 in the writings of Montaigne:
[Talking about Seneca and Plutarch] Their teaching is the cream of philosophy and presented in a simple fashion and pertinent.
Leur instruction est de la cresme de la philosophie et presentée d’une simple façon et pertinente.
Unfortunately, I can’t find exactly when the form crème de la crème first appears, but I’d be willing to guess it goes back a long time.