Chocolate Snickerdoodles

I recently borrowed from the library Baked In America. It's an absolutely gorgeous cookbook full of biscuits, cakes, brownies and other snacks and desserts, and it's gone to the top of my wish list (seriously, if you know me in real life, check out my Amazon.co.uk wish list. I want this book for my birthday)
Naturally, I had to make something from this wondrous book before returning it to the library. How could I resist chocolate snickerdoodles?
I'm quite pleased with them, except for one thing - tartaric acid is not cream of tartar!