Alexander
Joe Clay (Jack Lemmon) / Kirsten Arnesen (Lee Remick)
Teetotaler Kirsten says she doesn't like liquor but loves chocolate, so Joe orders her a Brandy Alexander, the start of her slide into alcoholism.
Sources: Wikipedia: Brandy Alexander; Wikipedia: Days of Wine and Roses (film); Cheryl Charming (Miss Charming): Cocktails in Film