The difficult part in determining treatment for chocolate stains (and chocolate ice-cream) is that chocolate contains milk protein as well as cocoa. Find pepsin/papain digestive enzymes (available in health food stores) and grind them up, adding enough water to make a paste. Rub it on the stain and let set for a few hours before rinsing.
If you still have a stain after removing the protein, try soaking in 3% hydrogen peroxide. Then wash as directed by manufacturer.