I'm looking for a specific type of manga, but I can't seem to find it anywhere, and I do remember reading it somewhere. It's a yaoi/shounen ai manga about two boys, one of them is popular and very good with magic, the other is not. The latter finds a forbidden spell and makes a love potion I believe he is going to use on a girl he likes, but instead uses on the boy, who "falls in love with him". I believe it was a oneshot...

Maybe Mitsu no Tsumatta Joushi 2nd story? It doesn't quite fit... You could try the "Love Potion" tag (I have read all of the yaoi mangas tagged there, but I don't remember any of them with a story like this).

I believed I've read that story as well, but the one I'm looking for is of the fantasy genre while this is more 'scientific'.

Kindan Spell?

Yes, thank you! ^^