Study author Dr. Farzaneh A. Sorond of Harvard Medical School in Boston said the study involved 60 people with an average age of 73 who did not have dementia.
The participants drank two cups of hot cocoa per day for 30 days and did not consume any other chocolate during the study. They were given tests of memory and thinking skills. The study participants also had ultrasound tests to measure the amount of blood flow to the brain during the tests.
"We're learning more about blood flow in the brain and its effect on thinking skills," Sorond said in a statement. "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."
Of the 60 participants, 18 had impaired blood flow at the start of the study. Those people had an 8.3-percent improvement in the blood flow to the working areas of the brain by the end of the study, but there was no improvement for those who started out with regular blood flow, Sorond said.
The study, published in the journal Neurology, also found people with impaired blood flow also improved their times on a test of working memory, with scores dropping from 167 seconds at the beginning of the study to 116 seconds at the end.