In "Lolita", Stanley Kubicrk preserved a  casual line from Nabokov's script, related to "bacon". When, in his adaptation of "Despair," Fassbinder mentions a brain core as soft as the Turkish delight, though, it's difficult to ascertain if it derives from Stoppard's inventiveness, Fassbinder's or Nabokov's ( a "Turkish delight" has been mentioned at least twice by Nabokov, in "Pale Fire" and in "Ada," whereas Tom Stoppard used it in "The Real Inspector Hound." 

However there's a more important speech in "Despair" (unrelated to dietary matters) when Hermann makes a Freudian slip of the tongue. He confuses the words "murder" and "merger." I cannot discover who coined the pun: was it Stoppard? Nabokov? It's a very clever maneuver in a story that teems with all sorts of murderous doublings, desintegrations and splittings. 

My hunch is that it was invented by Stoppard.