I highly recommend Yumi Hana, which is located at:

Schützengasse 7
8001 Zürich
Tel. 044 211 57 57

It's a fairly small restaurant (maybe 12 or so seats), but they also do take-away. In summer, they have "alfresco" tables outside.

The food is quite good - they have most of the popular Korean dishes. During lunch hours, it can be quite busy, so you sort of have to "eat and run" so that other people can sit down.

In addition, the back part of the restaurant sells a wide variety of Korean and Japanese food at very reasonable prices.

Very yummy!