Welcome! My name isXXXXX am a UC Davis graduate, and currently, a Medical Director of a veterinary hospital.
I am sorry to hear about this concern.
Well, it is already well over 1 hour, so we cannot induce vomiting to help out.
Fortunately, these are cupcakes. It's not a solid piece of cocoa powder or dark chocolate. So, it is quite possible that nothing may happen at all. Also, the larger Maddy is, the less likely for complications. If over 20 pounds, better to be over 40, her chances for toxicity drop a lot.
Mild symptoms of chocolate problems would be only vomiting or diarrhea. Diarrhea could happen anyway because of other cupcake ingredients. But! If you observe any tremors, shaking, or the heart rate seems really fast (you can feel just by grabbing over the chest), get into any vet for immediate care. A heart rate over 150 would be worth veterinary examination.
Here are some extra links, just in case you like extra details as well:Chocolate Toxicity Chocolate toxicity 2 Toxicity Calculator
The last link above is a very nice interactive site to help you understand the toxic effects of different types of chocolate depending on a pet's body weight.
I hope that information has been helpful.
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