Thank you for the additional information.
I have good news for you! Chocolate can be toxic
to dogs though Boof at 20 kilograms would need to eat about 400 grams of milk chocolate to have signs of chocolate toxicity such as vomiting, diarrhea, tremors, increased heart rate and hyperactivity. Since the muffins weighed less then 360 grams and the chocolate amount in the muffins was much less then the weight of the muffins, I do not see any risks of chocolate toxicity for him. If anything he may develop a mild digestive disturbance from eating something that he is not used to eating therefore I would watch him for any diarrhea or vomiting over the next 24 hours.
If Boof does have any vomiting or diarrhea, I would recommend holding off of food for about 8 hours. This will allow his stomach
some time to settle down. After 8 hours I would then feed him a bland food such as boiled boneless chicken
breasts mixed with some cooked rice for a couple of days.
Overall you do not need to induce vomiting tonight as he really should be just fine.
Hope this helps to answer your question - if not or if you have any more questions please feel free to reply by clicking on the "reply to" button. Thank you for your question!