Dori just did the same thing!! When I noticed that she wasn't in the same room as me, I went looking for her and found her under my daughters bed with an empty candy bar wrapper!
I flipped out and called the vet. The vet said that Semi sweet chocolate was much worse than milk chocolate. He looked in the "vet book" and found that for a 6lb dog 4 ounces of chocolate is TOXIC. Luckily the solid milk chocolate cupid that Dori ate was 1.2oz ! He did say that 1.4 oz would make them sick though. Sure enough, Dori puked what looked and smelled like melted chocloate for about 3 hours!!
But she is all fine now, Thank God!!