Letter of Interest For Deferred Students

Hey all,
I was recently deferred early decision, I wanted to write a follow up letter to show continued interest.
Just wondering WHEN I should submit the letter, and to WHO? Admissions office or specific admissions officer?

Thanks!

Submitted mine towards the end of Jan./ beginning of Feb. Sent it to my regional admissions officer (but I don’t think this second part mattered very much).

Early January to the admissions office according to brown.alumni.edu: “Assuming you remain interested in them, it is vital that you a) keep your grades up and continue to be involved in the things you love to do, and b) follow up with a strong letter of continued interest to their admission office, sometime in early January. Especially with Early Decision II plans now offered at many colleges, admission officers are aware that a good number of deferred students will apply ED II someplace else. So you need to let them know—in a brief but positive and heartfelt letter—that you are still interested and, if it’s the case, that they remain your first choice institution.”