Nice way to tell an ex that you don't want to cook for them any more?

My ex has a new girlfriend who is a professional chef, but she keeps asking me to cook her favorite meal for her. I don't want to cook for her any more. We are still friends, like found family really, but for the 9 years we were together, I was the cook in the relationship 100% of the time, and I'm still annoyed about it (we broke up 4 years ago). I've told her a few times that she should have her new pro chef girlfriend cook for her, but she keeps asking. Also pro chef girlfriend. What the heck! Is there some nice or gentle way to say "I won't cook for you any more" without bringing up ancient history?