Is it
possible to write words which contain Allah's names as decoration on edible
items. For example, 'JazakhAllahu Khairan' on cakes / biscuits intended as
gifts to teachers?

Answer: 

Primarily, it is permissible to write Allah’s Ta'aala name on objects, coins, etc. where it not disrespected or insulted in any way or form whatsoever. (Fataawa Hindiyyah Vol. 5 Pg. 323) 

In the enquired case, one should avoid writing Allah’s Ta'aala name on edible items because it is made for cutting, biting and consuming with no intention of acquiring a cure or barakah from it. Eventually the name may even be left marred or smudged if half the Great Name is cut causing apparent disrespect. Mere words of thanks should be used to attain the objective.

AND ALLAH TA'AALA ALONE IN HIS INFINITE KNOWLEDGE KNOWS BEST

ANSWERED BY: Mohsin Khan 

CHECKED AND APPROVED BY: Mufti Muhammed Saeed Motara Saheb D.B. 

Islamic Date: ٩۱ ذو القعدة ١٤٤٠ English Date: 22 July 2019