"Hey $BOB,
Thanks for taking the time to meet with me yesterday. I really enjoyed our conversation. I'm very excited about the opportunity to work with you and the team. Let me know if you have any questions.
Regards,
$YOU"
But I don't think there's anything wrong with sending a very brief, succinct thank you email to your interviewer(s). And in one case, someone I know got a job at Cisco because she sent a well-written thank you letter to the interviewer, because it put her in their minds before they decided among 10 similarly-qualified candidates. Keep it short and on point, with maybe one sentence about something they mentioned when talking with you. There is definitely an art to writing a 2-4 sentence, non-awkward email.
Personally I think a hand-written note would be weird, I wouldn't do it.
This is not one of those things like wearing a suit to a shorts-and-sandals startup, where they say "Oh what a dweeb, he's totally a not a culture fit here." No one thinks badly of sending a simple thank you note. Don't write them a love letter or anything, but spend the 2 minutes to write a sincere custom note, and snail mail it if possible, or email it otherwise.
I would consider a hire at a grade higher, given they're grasping what in hindsight would be basic relationship building skills. I would consider recommending the candidate to a friend if they were otherwise unsuccessful.
But don't be insidious. You're cultivating a relationship and have already met in person, not selling a connection via LinkedIn.
A simple message is fine, written semi-formally. If I was interviewing for a party on the beach 'Hey Dude!' might work, but I'm not, I'm interviewing for a serious job that involves serious decisions and some level of tact. 'Hey Dude!' doesn't do that.
Nor does 'pls' or 'thx' as it leads me to assume someone is so busy with their life attending interviews they're too busy to type 'e' 'a' or 'n' and would not be interested in a further job to add to their schedule.
Sending more than one mail may also imply a level of desperation. If having heard nothing for a week perhaps the interviewer doesn't share your level of conscientiousness and their position best skipped for someone able to reply to emails.
I also wouldn't handwrite it, unless a physical personal touch was make-or-break.
I doubt any hiring manager would not smile seeing a short proactive message in their inbox the day after an interview.
[I did receive, at the end of the interview, an invite to a Halloween party. She became a fantastic member of the team. I don't recommend Halloween party invites a solid interviewee strategy, however.]