Short ribs, cut thinly lenthwise against the bone. It is also labeled Flanken in non Asian grocers (not to be confused with flank steak, although that is a fine substitute.)

Skirt steak would be the best cut, although I have to admit that I submitted a food52 contest recipe using flank steak, which works very well but is more expensive. You can also use a flanken cut which is really cheap.

It is usually made with short ribs and you can use beef or pork.