“Choco Pies are an important mind-changing instrument,” said Andrei Lankov, author of The Real North Korea and a leading expert on the country.

“It has become a symbol of South Korean prosperity – and North Koreans read it. They are suffering and starving, but thanks to Choco Pies, DVDs and large-scale labour migration to China, people don’t buy the old story [that the South is even poorer] and the government does not sell it any more.”

The chocolate-covered, marshmallow-filled snacks enter the North through the Kaesong industrial complex, which is run jointly with the South – although Pyongyang has pulled out its 50,000-plus workers.

Because South Korean bosses were not allowed to offer cash bonuses, which were considered too capitalist, they turned to informal incentive systems. Instant noodles and mixed coffee sachets are also popular, but Choco Pies are resold for three or four times their original price and have achieved almost legendary status among North Korea-watchers.