This is Dr. Christian.
Thanks for the information. That's one sneaky dog. Luckily it would take a large amount of chocolate to affect a dog her size. Much more than what is present in the cookies so you shouldn't have anything to worry about. Here's a link with more about chocolate toxicity: http://www.petplace.com/dogs/chocolate-toxicity/page1.aspx
The worse case scenario is probably vomiting and diarrhea. These may develop over the next day or so. I would feed her a regular meal of dog food right now to "dilute" out the cookies and then monitor her closely. If you have more questions please let me know.