Prepare pork belly by removing the skin and separating the lean and fatty parts. Cut into small dice.
Finely chop scallions and set aside.
In a bowl, mix dry yellow bean paste, sweet bean sauce, sugar, water, and soy sauce until combined. For enhanced flavor, huadiao wine may substitute water, but avoid cooking wine.
Heat three tablespoons of oil in a wok over medium-low heat. Add diced fatty pork and stir-fry until lightly browned and the oil turns clear.
Pour the mixed sauce and half of the chopped scallions into the wok, stirring evenly.
Add all the lean pork, mix well, and simmer over medium-low heat for 30-40 minutes, stirring occasionally.
Once the sauce has thickened, incorporate the remaining scallions and stir-fry for two minutes. The zhajiang sauce is now ready.
The finished sauce will have clear oil on top, which can be poured into a clean bowl while hot for later use in cooking.
For storage, transfer unused sauce to a clean container, top with some residual oil to form a protective lard layer after cooling, preserving freshness.
Serve by boiling noodles, adding shredded cucumber, radish, blanched bean sprouts or soybeans, and mixing with the zhajiang sauce to complete authentic Beijing Zhajiangmian.