Naoko Otsuka didn't think twice about paying the equivalent of $23 for 141.7 grams of chocolate. The dark brown slivers of sweetness weren't just a treat, the Japanese homemaker says, but an opportunity to learn more about a superfood.

The six bars Otsuka bought — each from a different batch of cocoa beans grown in Vietnam — were made by an artisanal manufacturer and sold at a store in Tokyo's bustling Shibuya shopping district.

"I want to taste the difference between each bar," says Otsuka, 55. "Before coming to this shop, I didn't know beans could taste different depending on their origin."