I struggled with this as well. I wrote a psychological thriller and everyone is a suspect and everyone has secrets. I didn’t want to “hide” this from my readers since that would be doing a disservice, so I’ve decided to create a short story around the protagonists mother (who is a prominent character in the main book), but it’s from an outsiders perspective. The “cold” reader will get a thrillery short story which will hopefully lead them to wanting to know more about the mother in the main book, and the “warm” reader will get an easter egg since after reading the book they will know more about what is happening in the cookie than their “cold” counterparts, but still be able to enjoy the extra story.