Chocolate is a neuro toxin. It can cause tremors and seizures. The amount of chocolate needed to cause these symptoms varies individually.
However milk chocolate is less than 10% chocolate. Most cats who eat this form of chocolate can be hyper form the sugar and developed a case of diarrhea from the other ingredients.
If your cat has eaten a lot of bakers chocolate he may need to be placed on IV fluids to help flush the toxins from his system.
If he ate a small amount of milk chocolate watch for trembling or unsteady walking. Vomiting or diarrhea that does not resolve after a 12 hours is also a sign that the cat needs to see a veterinarian.
From this point on keep the chocolate in a location that is not accessible to the cat. Once a cat associates the smell with the taste they become hooked. And remember they can smell much better than we can.
I hope this information has helped