Luini is an historic place to eat a good panzerotto. The queue is always very long but don't be scared, it's also fast! http://www.luini.it/
Because it's a cheap and fast lunch in a place where locals go too. And personally I love it, panzerotti there are soooo delicious! And it's open since 1888, so a real Milan experience!
there's no place to sit, you'll have to search around a bench or something else.