Anker Pallesen and his wife Ella worked out the formula at home, mixing milk, cocoa and sugar until they had something they thought people would buy. They were right, though not immediately - the first factory turned out a thousand bottles a day, which is a rounding error by the standards of what the company became. Esbjerg was the logical place for it. Denmark's great North Sea fishing and export port sat in the middle of Jutland's dairy country, with milk on one side and ships on the other. In 1964 production moved to Bavnehoj near the harbour, into a building that had gone up in 1894 as an epidemic hospital - an unglamorous inheritance for a factory whose product would end up on every second street corner in the country.

The quiet engineering achievement of Cocio Classic is shelf stability. The drink is homogenised, so the cocoa solids stay suspended rather than settling into sludge at the bottom, and sterilised, so it keeps at room temperature. That is why a hot dog stand with barely any cold storage can carry it, and why it travels. The composition is unfussy - milk at around two percent fat, sugar, and certified cocoa - and it has been left alone for seventy-five years while the packaging around it changed repeatedly. The first can appeared in 1974. A 0.4-litre bottle followed in 1983, and a full litre bottle in 1986, which is the point at which Cocio stopped being a snack you bought one of and became something families kept at home. The first Danish television commercial ran in 1986, timed to the new litre bottle.

Cocio was not alone for most of its life. A rival brand called Congo held about a quarter of the Danish chocolate milk market, and the two competed until 1989, when Cocio bought it. After the takeover there was no meaningful competition left - one company, one national habit. Ownership of that habit turned out to be more complicated than the market share. In 1976 Anker Pallesen sold the company to the American Borden Food Corporation, though production stayed in Esbjerg. It took until 1999 for it to come home, when two Danish concerns, E. Bank Lauridsen Holding and the IAT group, bought it back. Arla Foods took a fifty percent stake in 2002 and became sole owner on 1 January 2008. Around the turn of the millennium a new plant opened at Kjersing, north of Esbjerg, capable of turning out 800,000 bottles and cans a day.

Then there is the strangest chapter, which has nothing to do with chocolate and everything to do with customs law. The United States effectively does not admit foreign liquid milk; the Federal Import Milk Act allows it only as a minor component of something else, such as infant formula. Denmark had been trying to get around this for over thirty years. In 1998 an importer named Jamin Potamkin entered Cocio into an FDA pilot programme, and it worked - the first time a foreign milk was permitted into the United States and distributed in all fifty states in drinkable form straight from the container. When Danish dairy interests asked him to do the same for their yoghurt, the answer from both Potamkin and the federal government was no. The arrangement ended in 2003. Cocio was briefly imported again by another company, but American consumers cannot buy it today.

Cocio still travels, if not to America. It sells across Scandinavia and turns up in Spain, the United Kingdom, the Netherlands, Poland and the Philippines, and it retains a small following in New England, a leftover from the years when it was legal there. At home, the brand did something unexpected in 2010 by hiring Eva Mendes as the face of its advertising. The campaign worked to a degree nobody quite predicted: annual profits went from roughly three million kroner to over ten million. It is a curious thing to hang a Hollywood actress on a product whose entire appeal is that it tastes exactly the way it did when your grandparents drank it standing at a sausage cart in Esbjerg. But the recipe was never going to change, so something else had to.