Both are OK. Using 'a' doesn't refer to uncountable nouns here. " is more proper..
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
CoincidenceThe structure is :
with countable nouns :
He is such a good boy.
with uncountable nouns :
it was such cold water we couldn't drink it
with plural nouns :
He had such dirty hands that he couldn't clean them
I don't understand why we should say "such a good coffee", if 'coffee' is uncountable...
I think it results