I've posted an answer to a related question once. Let me quote the related part again here:
14 adjectives (3): position after as, how, so, too
After as, how, so, too and this/that meaning so, adjectives go before a/an. This structure is common in a formal style.
as/how/so/too/this/that + adjective + a/an + noun
I have as good a voice as you.
(Practical English Usage by Michael Swan)
It's a normal as ... as pattern as Stephie said.
What's probably not normal for many learners is the pattern adjective + a/an + noun.
But if we think about it, it's probably not that strange. For example, I'm sure that you're fine with this one: I have as many books as you (have). It could be a little tricky for learners when the noun inside of as ... as is singular, but It's just that we don't hear it as often as as + adjective + as (e.g., This book is as good as that book).
(To the Dutch, milk and cheese are) as essential a part of the weekly shop as rice is for a Chinese shopper or teabags are for an Englishman is perfectly grammatical in English.