I can do them one better: my family has a meta -secret recipe. It didn't used to be secret, but it became secret through no effort or intent of our own. For a long time, decades at this point, my mom has been known for the brownies she brings to events (and, by extension, that my sister and I bring to events). People would rave about them and demand the secret, which my mom, my sister, and I would all freely give out…
so I just use it and people love it, but I literally can't tell them or they'd think I'm serving them rubbish