Use of partitive before a Singular Adjective preceding a Singular Noun

Alan, do you have any reference for “c’est de bon gâteau”? being accepted grammar?  I think it is incorrect.

If heard, I would assume the speaker was using “c’est” instead of “ce sont”, as occurs in speech but not usually in writing and that it was “C’est (ce sont) de bons gâteaux”. 

De is used with plural adjectives preceding nouns, and with negations or quantities (of). Laura Lawless gives the specific example of saying “je veux du bon fromage”. 

My most reliable reference, my wife, thinks “c’est de bon gâteau’ is incorrect, and would never use it. Of course, I know she isn’t always right, but I would never say she’s wrong! (Especially as she is sitting beside me at the moment)

partitive_article

Interesting question. I think "gâteau", like "cake" can be used countably ("a cake"), or uncountably ("some cake"). 

According to the following lesson, "des" changes to "de" before an adjective. It only talks about plural nouns, but I think it can optionally be applied in this case.

"Des" becomes " de/d' " in front of adjectives preceding nouns (French Partitive Articles)

So I believe all your examples are possible:

C'est un bon gâteau. = This is a good cake.

C'est de bon gâteau. = This is good cake. (literary/elevated? French)

C'est du bon gâteau. = This is good cake (normal? French)

You'll find the rule that all partitives change to "de" before an adjective in old grammar books, e.g. "French: How to Speak and Write It" by Joseph Lemaître

https://www.google.co.uk/books/edition/French/YiPEAgAAQBAJ?hl=en&gbpv=1&dq=de+bon+g%C3%A2teau&pg=PA193&printsec=frontcover

Grevisse says that "de" is still found in writing, although "du" etc. is more normal. See the bottom of page 820 in this link: tinyurl.com/fvh9rt

Thanks Alan. Larousse does indeed list it as “vieux ou littéraire” only.

https://www.larousse.fr/dictionnaires/francais/de/21718#difficulte