The sauce is made with black bean paste, called chunjang (hangul: 춘장; hanja: 春醬), to which are added sauteed diced onion, zucchini, and ground meat (either beef or pork) or chopped seafood. When cooking the sauce, cornstarch is typically added to give it a thick consistency and it is served hot over noodles. The paste, which is made from roasted soybeans, is called chunjang (literally "spring paste") when unheated, while the heated sauce (containing vegetables and meat or seafood) is called jajang (literally "fried sauce").
In Korea, singles eat jajangmyeon on Black Day. The noodles are a comfort food for those who have no significant others.
The Black Day (April 14) is an informal tradition for single people (a.k.a. Unit Solo, Korean: 솔로부대) to get together and eat noodles with black bean sauce in South Korea, sometimes a white sauce is mixed because those celebrating the Black Day didn't experience the White Day. The idea is that those who didn't give or receive gifts on Valentine's Day or White Day can get together and eat Jjajangmyeon (짜장면), white Korean noodles with black bean sauce (hence the name), to commiserate their singledom.
Depending on individual tastes, the recommended amount of water can be added for a typically soupy base.
However, adding less water can create a far thicker sauce wich stick to noodles themselves for a fried style.
Though harder to achieve perfection without practice, this 'dry' style is favoured by many.