一 + 脸 + Adjective + 的 + 样子
The word 脸 (liǎn) normally means "face," but it also functions as a classifier for facial expressions. In the pattern 一脸 + adjective, it describes someone's entire face showing a particular expression, as in 一脸得意 (a smug expression).
她一脸开心地跑过来告诉我好消息。
tā yì liǎn kāixīn de pǎo guòlái gàosù wǒ hǎo xiāoxi.
She ran over with a happy face to tell me the good news.
老师一脸严肃地看着我们。
lǎoshī yì liǎn yánsù de kànzhe wǒmen.
The teacher looked at us with a serious expression.
他一脸惊讶,好像完全没想到。
tā yì liǎn jīngyà, hǎoxiàng wánquán méi xiǎngdào.
He looked completely surprised, as if he hadn't expected it at all.