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I am sorry Chester has eaten this chocolate. I suspect that he has eaten a toxic amount of chocolate. Anything above 1.25g per kg of body weight of dark chocolate requires decontamination. So if it is more than 30g of this chocolate, Chester will need to get to a vet. They will need to induce vomiting and provide something like activated charcoal to prevent absorpation of the chocolate form the intestines. Ideally this can be performed within 4 hours on ingestion of the chocolate.
I hope he is OK
I just noted a typing error in my answer, I was meant to say absorption of the chocolate from the intestines. Sorry for any confusion.