It has to be dark chocolate, it will not work with milk chocolate, that is about as much as I know.
It has however been medicaly proven that a square of Dark chocolate and a glass of red wine each day can be good for your heart, its just that when I did it you could say I kinda overdosed a little.
What I can remember of it is that I was uber alert, could not sleep and my heart felt as if it was about to explode. At the end of it, it felt so bad, my head was cold and numb and I slept for 24 hours solid.
If you doubt its effect, its cheap enough to try it for yourself. Dark chocolate only with the highest cocoa % you can get your hands on. I will however not be held responsible for any fatalities