My dog ate a cupcake with chocolate icing!!

My dog ate a cupcake with chocolate icing, she grabbed it right off the counter when we weren't looking. I am not sure how bad they are for dogs whether this is serious or I should just look foward to cleaning up throw up. She is 45 lbs so she might be able to handle it a little better than a small dog. I am just not sure how much chocolate a dog can eat before you have some big time worry!