I would not bring the plate of cookies. I was in graduate school (U.S.) and in our department we were expected to bring a small set of treats for our PhD defense. Years later, my wife was doing her master's defense in the math department. Despite her reluctance, I brought a similar set of treats, but the committee rejected them, not wanting to be 'bribed'. Thankfully, she got her master's degree anyway!
As for your interview, definitely do send email thank-you notes, and if it looks like a decision will take a while, postal thank-you notes can help and make a difference! This once worked for me.
Remember, occasionally they will hire you right away, but sometimes it can take almost a month. In the latter scenario, yes, you probably weren't their first choice, but taking such a job is often needful, and yes, I've done that myself, also.