Recruiting interns: interviewer side

I was put in the same position as you back in the fall. I would definitely do it via email,I typically responded to the thank you email with

"We appreciate your interest in the position, but have decided to move forward with another candidate. Best of luck with your future endeavors".

If they asked for feedback after that, I would provide them with some ways to better handle questions next time

Thanks. I was debating whether to include feedback in my message, but I'll provide it to only those who request it.

In my case, HR usually called to reject, even if the interviewer was an analyst.

It is pretty standard procedure for HR to call to make a call to reject (assuming you're at a bank large enough that you have a dedicated HR department).

...

Thanks everyone for the contributions. As mentioned above, I decided on an email rejection, with possibility of giving feedback to those who would ask for it.

Also two points of advice: - have your motivation story straight (why banking, why this bank, why this team); you don't imagine how many good candidates, with impressive education and previous experiences cannot give a concise and structured response - send a Thank you & reiterate interest note; you will definitely stand out of the crowd. In our case, only 20% of candidates did it