Egg Coffee is the „Cappuccino of Vietnam“. Let me quote the book I found on it in one of the hostels but unfortunately I only took pictures of the information on egg coffee and not what the book is called.

The history of the drink began in 1925, when a young Hanoian man named Nguyen Van Giang started to work as a bartender in the Metropole, a five-star hotel in the French Quarter. Every day, he made cappuccino for his wealthy guests. But due to the scarcity of fresh milk at that time, the ingredients were too luxurious for most Vietnamese people to try even once.Mr. Giang dreamed of creating a version that he could happily serve his countrymen; then in egg coffee or cappuccino 1946, the "Cappuccino of Vietnam" was born. Mr. Giang mixed coffee, a little condensed milk, a dab of butter, and two raw egg yolks from free-range 'running chickens' as they are called in Vietnamese. This is important as the recipe requires the eggs to be of a high standard and only uses those fed on fresh food and not battery farmed. However, the secret ingredient that makes the egg part so tasty still remains unknown today.

P.S. In the introduction the author mentions that when he first heard about it he felt absolutely disgusted on that idea. and I just love the pure honestly, I felt exactly the the same. And he was positively surprise, when he finally tasted it, which I can also just agree on!