WASHINGTON (AFP) - Globe-trotting US diplomat John Kerry is known for his sweet tooth. But he surprised top chefs this week by revealing he loves to cook, and once ran a bakery.

Back in the 1970s when he was "bored stiff" working in a private law practice in his home town of Boston, he and a friend hit upon a madcap idea to open a late-night cookie shop.

"We came out after dinner, and I was craving a chocolate chip cookie. And this has been a craving all my life," Kerry joked in the sumptuous surroundings of the State Department at an event attended by some of America's top chefs.

"So we said, 'Wow, there's nowhere in Boston down here where we can buy a great cookie late at night.'"

After scouting around, Kerry and his then partner found a spot at the Faneuil Hall Marketplace, negotiated a space and two months later were ordering food mixers and "a fancy new stove" for their business adventure.

"I was having these great visions of 40 stores in 40 cities very quickly, and I suddenly realised, 'You know what? I don't have any menus. I don't have any recipes'."

Addressing US chefs who will take part in America's contribution to the 2015 Milan Expo, Kerry revealed he began whipping up batches of cookies on his home stove "as many different chocolate chip versions as I could make."

"Lo and behold... by cribbing a little bit off the back of" a packet of cookie dough and "working very cleverly with molasses and pure Lindt chocolate and the best butter we could find and so on, we made a great cookie."

The Kilvert & Forbes Bakeshop opened on time and within a year had won a Best of Boston award.

Although Kerry and his partner have long since sold his shares in the business, the store still exists and the former Massachusetts senator remains one of the best customers of its chocolate chip cookies.

And he said the experience had been a good lesson in how "people connect in unique and powerful ways over food" something which he has used in his diplomacy.

The chefs who will take part in the Milan Expo opening next month are "as committed to American diplomacy as any group of people I've ever met", Kerry said.

"We're going to showcase American innovation and technology and promote our agricultural products and trade," he added.