Hello,
This is Dr. Christian.
It's been 15 minutes since she ate it.
We don't believe there were any artificial sweeteners in it.
Good. Luckily it would take a lot of chocolate (almost a pound of pure chocolate) to cause a problem in a dog this size. The worse case scenario will probably be vomiting and diarrhea. You can give her 2 tablespoons of hydrogen peroxide to get her to vomit up the cheesecake. If she does not vomit in 10-15 minutes you can repeat it. If she starts to vomit or have diarrhea then take away her food for 8-12 hours and just offer water or warm chicken broth. After letting her stomach rest you can then offer boiled chicken breast, white rice and scrambled egg whites. These are easy on the stomach. If you have more questions please let me know. Here's a link about chocolate toxicity:
http://www.petplace.com/dogs/chocolate-toxicity/page1.aspx
Dog Veterinarian
12 years of veterinary experience in canine surgery, medicine and behavior