A study of 60 elderly people with no dementia found two cups of cocoa a day improved blood flow to the brain in those who had problems to start with.
Those participants whose blood flow improved also did better on memory tests at the end of the study, the journal Neurology reported.
Experts said more research was needed before conclusions could be drawn.
It is not the first time cocoa has been linked with vascular health and researchers believe that this is in part due to it being rich in flavanols, which are thought to have an important role.
In the latest study, researchers asked 60 people with an average age of 73 to drink two cups of cocoa a day – one group given high-flavanol cocoa and another a low-flavanol cocoa – and consume no other chocolate.
Ultrasound tests at the start of the study showed 17 of them had impaired blood flow to the brain.
There was no difference between those who drank flavanol-rich cocoa and those who had flavanol-poor cocoa.
But whichever drink they were given, 88% of those with impaired blood flow at the start of the study saw improvements in blood flow and some cognitive tests, compared with 37% of people whose blood flow was normal at the beginning of the study.
MRI scans in 24 participants found that people with impaired blood flow were also more likely to have tiny areas of brain damage.
“We’re learning more about blood flow in the brain and its effect on thinking skills,” said study author Dr Farzaneh Sorond a neurologist at Harvard Medical School.
“As different areas of the brain need more energy to complete their tasks, they also need greater blood flow. This relationship, called neurovascular coupling, may play an important role in diseases such as Alzheimer’s.”
The researchers said the lack of difference between the flavanol-rich and flavanol-poor cocoa could be because another component of the drink was having an effect or because only small amounts were needed.
Dr Simon Ridley, head of research at Alzheimer’s Research UK, said this was a small study but that it added to a wealth of evidence.
“A cocoa-based treatment would likely be very popular, but it’s too soon to draw any conclusions about its effects.
“One drawback of this study is the lack of a control group for comparison, and we can’t tell whether the results would have been different if the participants drank no cocoa at all.”
But he added: “Poor vascular health is a known risk factor for dementia, and understanding more about the links between vascular problems and declining brain health could help the search for new treatments and preventions.”