Cocoa powder, but concentrations very widely depending on the processing. Milk chocolate has most epicatechin processed out of it.
“You would have to eat a large amount of chocolate”, along with its fat and calories, said Hagen Schroeter, director of fundamental health and nutrition research for Mars, which funds many flavanol studies and approached Small for this one. (“I nearly threw them out”, said Small, who added that he later concluded that the company employed serious scientists who would not bias the research.)