"Chocolate bars with higher "% cocoa solids" generally have more polyphenols (flavenols) than ones with lower content." When researchers analyzed 46 different bars, the dark chocolate bars had more polyphenols than milk chocolate, and polyphenol content was higher in bars with increased percentage of cocoa solids. "Even small amounts of dark chocolate (6 to 10 grams--or 0.2 to 0.35 ounces--a day) have been linked to cardiovascular benefits."
Summarized in The University of California, Berkeley, Wellness Letter, Oct 2015: Volume 32, Issue 1. Original study appeared in the Journal of Functional Foods.