The New York Times usually doesn't just make stuff up, so I tend to believe their claim that Gwyneth Paltrow used ghostwriter Julia Turshen to help write My Father's Daughter, a cookbook featuring her family's recipes that was published last year:
Julia Turshen, who is writing a second cookbook with Gwyneth Paltrow after their collaboration on My Father’s Daughter, began as the ghostwriter for the ghostwriter on a book by Mario Batali, tagging along with a notebook as the chef filmed a culinary romp through Spain.
Gwyneth, on the other hand, believes that the New York Times just made this up. She tweeted on Saturday:
Love @nytimes dining section but this weeks facts need checking. No ghost writer on my cookbook, I wrote every word myself.