The locavore Brooklyn siblings behind cult chocolate company Mast Brothers are bringing their small-batch business abroad.

Mast Brothers, started in Williamsburg in 2007 by the bearded Michael and Rick Mast, has been quietly expanding in recent months. The company, which recently built a second factory next to the Brooklyn Navy Yard, is now constructing one in London.

"We're already selling our chocolate bars all throughout Europe, but we never want to be just another chocolate bar on a shelf," Rick Mast said in an interview Saturday.

The new factory will be in London's Shoreditch neighborhood, allowing Mast Brothers to bring its detail-obsessed--and delicious--production process closer to its European buyers. It's a bold move for the Iowa-raised brothers, who are betting that their decidedly non-Hershey's version of American chocolate can compete on the continent known for perfecting the candy. 

The international expansion is also a massive step for a company that has built its reputation on being an artisanal, sustainable, "bean-to-bar" control freak about making chocolate. Mast Brothers sources its organic cocoa beans from locations including Belize and Papua New Guinea, and ships them to Brooklyn by sailboat. There, it roasts, shells and grinds the beans together with just cane sugar, ignoring the palm oils and other additives found in much commercial chocolate.

The brothers' painstaking process involves everything from hunting down the best cocoa pods to building the factories' own special equipment out of motorcycle jacks and yoga mats. The result is expensive, at around $10 per bar, but intensely flavorful--and endorsed by top chefs like Thomas Keller.

"We want to return chocolate to the world," Rick Mast told an audience at the start of a two-hour demonstration and tasting event Saturday for the World Science Festival.

"In this era of food, we know the name of the cow you're about to eat--but not with chocolate," he added.

A former restaurant chef, Mast said he spent a lot of time experimenting with how to roast the organic cocoa beans--stacked in burlap sacks against one side of the new factory--in the convection ovens lining the other side.

Like steak, "we kind of cook our beans medium rare," Mast told the group, adding that "medium rare" in this case comes out to 15 minutes at 300 degrees Fahrenheit, more or less: "Everything's better medium-rare."

On Saturday, Rick (without his brother for the day) and Harvard-trained physicist Naveen Sinha walked visitors through the new two-story factory in Brooklyn's Clinton Hill, about two miles south of the company's original Williamsburg location. Now Rick and Michael are preparing to travel more than 1500 times that distance to oversee their third factory.

The New York-London leap raises an obvious question: how does a company that thrives on its two founders' obsessive attention to detail survive a transatlantic expansion?

For Mast and his brother, the solution lies in good hiring--and a lot of commuting.

"It starts with culture ... finding awesome people who are going to be passionate about chocolate as a food," he told me on Saturday.

Also, "we'll definitely be flying back and forth a lot--and it's great that there are two brothers," he added. "We're really excited. We've thought about doing this for a long time."