Le Ménagier de Paris, written near the end of the century was arguably the first cookbook written as a set of instructions for a second party to use when managing a third party, in this case, for the young wife of an elderly gentleman to use as a guide for household management including supervising the cook.
It's not indicated well here in the text, but this was written in 1393 according to the footnote.
Le Ménagier de Paris, 2 vols (Paris: the author, 1393; repr. Paris: Jerome Pichon, 1846)
The text source comes from 