AU and EN chocolat

Question

I think there is a mistake with the sentence possibilities: "My mother is taking care of the chocolate log".
Possible answers: "Ma mere s'occupe de la buche AU chocolat". "Ma mere se charge de la buche EN chocolat". and "Ma mere s'occupe de la buche de Noel EN chocolat" and "Ma mere se charge de la buche de Noel AU chocolat".
Why some have EN and other sentences AU? I dont see the difference. Shouldn't all the sentences be with AU chocolat?
Thank you

Accepted Answer

Solveiga, 
both expressions can be correct. 
‘ au chocolat ‘ is most common - in simple terms, it usually refers to a rolled cake log, with a chocolate ganache (although as in all things cooking, re-imagining the traditional is common )
‘ en chocolat ‘ is usually reserved to describe a variant made entirely, or predominantly, with chocolate. 
As with all things French and cooking, everyday usage can vary from ‘ grammatically pure ‘ definitions. 
Searching the terms on the net will yield further examples, but 2 illustrative links below :
 https://www.meilleurduchef.com/fr/recette/embout-buche-chocolat-moule-transfert.html 
https://www.sortiraparis.com/actualites/noel-et-fetes/guides/265402-les-meilleures-buches-de-noel-au-chocolat-2023-a-paris