During a visit, my mother’s and my dog ate some chocolate cake that the wind blew off a table. We notice it in 10-15 mins, vet said just bring them to induce vomiting. We did that, they got an injection each, and vomited out everything soon.

In practice, vets want to take no risks by calculaing/guessing exact cocoa content, the best is just to have them vomit as soon as you can.