Help a Buddy Out: Does Anyone Know What Series This Is?

I was texting* with a friend yesterday and he said that he cannot remember the name of most of what he read in middle school. He was largely okay with that, but there’s one exception:

there’s only one series I desperately want to remember what it was.

Was sort of spooky mystery, 1950ish settling with some siblings always visiting this wise professor who always made them chocolate cake and they were always getting on the wrong side of evil magicians and eldritch powers.

It’s a vague amorphous memory that gives me super nostalgia and angst that I can’t remember anything beyond that image lol

Think back 15 years or so, does any one have an idea what he might have been reading? It’s not necessarily (probably not at all likely) YA. I can’t think of anything, and search engines aren’t working for me.

* I have to stress this conversation was in text–he’s a proofreader, and if it got out that he typed like this outside of a text environment, he’d probably stop talking with me outside of formal letters.