Why dark chocolate?
Any chocolate will do in an emergency, but getting your day started is all about administering the cacao drug. The stronger your chocolate,
The kick from dark chocolate is not caffeine - much. You need about a kilo (over two pounds) of good dark 70% choc to equal an espresso (unless of course it is chocolate with coffee in it: Whittaker's Dark Moccha just about blows my head off).
The really active ingredient is theobromine - the same stuff that will kill your dog if you are not careful. It may be rough on dogs, but it feels to me that theobromine is a much more pleasant kick than caffeine. Caffeine causes me anxiety, sweats, clammy skin, shakes... Chocolate just makes me feel good and get up and go!
The following list of benefits for dark chocolate comes from Leonore H. Dvorkin (also a proponent of chocolate in the morning). Being a bit of a skeptic, i don't know how much of this is really supported by scientific research and how much is bulldust, but when it comes to chocolate, who cares eh?
See Leonore's article for much more on the background to these.