When so is used with an adjective and a singular
countable noun, the word order is

so + adjective + a / an singular countable noun +
(that) + result / effect sentence:

That was so difficult a test (that) almost no one
passed it.

He gave so unusual an answer (that) we weren't sure of what he meant.

Sentences such as the above seem "forced" and
rather artificial, however, so most people use such,
not so:

That was such a difficult test (that) almost
no one passed it.

He gave such an unusual answer (that) we
weren't sure of what he meant.

So cannot be used with an adjective and a plural
noun: instead, such is used:

Those were such delicious cookies (that)
everyone ate two or three of them.

Ana and Brigitte are such fast runners (that)
they've never lost a race.