Eggs with your coffee? In Vietnam — a culture that loves caffeine at least as much as Americans do — locals put eggs IN their coffee.

First, a bit of history: Nguyen Van Dao says that back in 1946, his father Nguyen Van Giang, a barman at the Sofitel Legend Metropole Hotel in Hanoi, ran out of milk to serve with the coffee. In a pinch, he whipped up a mixture of egg yolks and condensed milk, and the rest was history.

As popularity for the beverage grew, so did the business to the Giang Coffee shop in the old quarter of Hanoi today.

The country isn't the only one to have used eggs in coffee. Several Scandinavian countries also crack egg yolks into coffee grounds, which supposedly making the coffee less bitter. The two versions could not be more different, save for the use of a shared ingredient, the egg. The Vietnamese version tastes like a foamy flan (a Spanish dessert) while the Scandinavian version is said to enhance the coffee, rather than change the taste completely.

The coffee, known locally as Ca Phe Trung, has been so successful that most cafes in Hanoi serve up a version of their own. Recently, CNBC visited the Giang Cafe in Hanoi, where tourists and locals alike lined the walls waiting for the beverage.

Blink and you'll miss the tiny entrance on the busy street of Nguyen Huru Huan of Hanoi's Old Quarter. Once inside, a tiny kitchen at the bottom of the stairs — where the smell of meringue wafts in the air — is where a chef whips the sabayon for the egg coffees into a frenzy. A mixer then blends the milk and eggs.

While other cafes are recreating the drink in their own kitchens, egg coffees themselves have become known as a city-specific specialty. This particular blend, however, is still considered unique to Giang Cafe.

Customers can have the egg coffee one of two ways: hot or cold, each about $1.15. Compared with street food in Hanoi, that price is actually on par with the cost of a banh mi sandwich or bowl of pho, Vietnam's staple noodle soup.