The key ingredient is flavonoids
A cup of enriched cocoa may help improve the working of blood vessels in diabetic patients, research suggests.
Doctors prescribed three mugs of specially formulated cocoa a day for a month, and found "severely impaired" arteries regained normal function.
The German study, featured in the Journal of the American College of Cardiology, suggests chemicals called "flavanols" may be responsible.
But charity Diabetes UK said eating more normal chocolate would not work.
People with diabetes are at greater risk of cardiovascular problems such as heart disease and strokes, partly due to the effects of high blood sugar on the linings of blood vessels, which stops them being able to expand as much when needed by the body.
This can result in higher blood pressure, which can then cause further problems.
While a healthier lifestyle can reduce the risks, it often does not solve the problem completely.
Cocoa naturally contains "flavanols", antioxidant chemicals which are also found in some fruit and vegetables, green tea and red wine, and has been linked with health benefits by other studies.
The type of cocoa used in the study cannot be found in the shops and is a version enriched with far higher concentrations of the chemicals.
Other studies are looking at whether flavanol-enriched chocolate could benefit patients.
Ten patients were told to drink the cocoa three times daily for 30 days, and a special test was used to measure the function of their blood vessels.
The ability of the vessels to expand in response to a demand for extra blood from the body appeared to increase almost immediately.
On average, a healthy person's arteries could expand by just over 5%, while the average of the 10 diabetic patients was just 3.3% prior to drinking their first mug of cocoa.
Two hours after drinking the cocoa, their response averaged 4.8%, and over the 30 days, this improved, to 4.1% even before cocoa, and 5.7% two hours after a mugful.
Dr Malte Kelm, from the University Hospital in Aachen, who led the study, said that the flavanols could be working by increasing the production of nitric oxide, a body chemical which tells arteries to relax and widen.
He said: "Patients with type II diabetes can certainly find ways to fit chocolate into a healthy lifestyle, but this study is not about chocolate, or about urging those with diabetes to eat more chocolate.
"Our results demonstrate that dietary flavanols might have an important impact as part of a healthy diet in the prevention of cardiovascular complications in diabetic patients."
A spokesman for Diabetes UK said the findings were "interesting".
"Flavanols do seem to offer potential health benefits for people with diabetes but, at this stage, we don't advise people to start drinking lots of hot chocolate as it can be high in sugar and fat.
"More research is needed in to the long-term effects of consuming such high amounts of flavanols."