The most formal I’ve seen are in legal contexts. I’ve seen things like:
Regarding your correspondence dated 9/28/19
Putting the date in adds formality and clarity to which letter you are referring to. Outside of a legal or business letter it would be strange to add the date that way though.
You could also start with “pursuant to”., like you said, but “further to” doesn’t sound natural. “ In response to ...” is another option.