Google Translate is not a valid source ;) Even if it got better recently, true. Technically it translated it correctly, "miej" is 2nd person singular of "to have". That's true, grammatically it's okay. But I can't imagine anyone saying that. Perhaps "Proszę, miej ciasteczka!" as in someone that really wants to eat some cookies now and hopes (prays, almost?) that their grandma has some cookies at home. Still rather unusual, though.
Generally... we'd just use some other construction here. Totally different. Maybe simply "Proszę, ciasteczka". Maybe I'd say that I bought/baked some cookies. The most probable to me is "Częstuj się/Poczęstuj się". I believe those would be translated into English as "Help yourself".
Actually, in a way... "Masz ciasteczka" could be used when English would say "Have some cookies". Maybe it's not the most polite way, but it can be said when you give something to someone.
Have (some) cookies! / Help yourself and have (some) cookies! - Jedz/Zjedz
(parę) ciasteczek!/Jedz/Zjedz (sobie) ciasteczka!/Poczęstuj się ciasteczkami!
You have cookies! - Masz ciasteczka! (it is about possession)
Take your cookies (and leave me alone)! - Masz (swoje) ciasteczka!
You have/eat cookies!/ You are eating/having cookies! - Jesz ciasteczka!
Standard English simply uses "you" no matter whether you speak to one person or more, Polish does make this distinction: "ty" for just one person and "wy" for more.
And then it affects the form of the verb - you already see that the pronoun "ty" ("you") is omitted, because the form of the verb makes it obvious what is the subject.
Because this is just a simple declarative sentence stating that you do indeed (already) have cookies.
"Have some cookies" doesn't translate well into Polish. I think the most natural is "Poczęstuj się ciasteczkiem", with a single cookie (even if you will take more). "poczęstować się" is... I don't know how to translate it. To take something that is offered to you and eat it?