Interesting question…..I’m wondering if the term really doesn’t have any relevance outside of ice cream? There’s a good chance that your customer doesn’t know what it means, either. I’m reminded of a group of us standing outside one summer evening and everyone talking knowingly about “heat lightning” we could hear. When I realized I really didn’t know what that was, I asked, and no one else did either. They were just using a term they had heard.
Anyway, if you connect “French Vanilla” with a custard + vanilla, then Rose’s yellow cake (all yolks) plus a Silk Meringue vanilla frosting is probably as French Vanilla as it can get.