Chop scallion and chives finely, set aside.
Blanche mungbean sprouts, onion, cabbage, and chop finely. Salt and let sit for 2 minutes, then place in a cheese cloth and squeeze all the water out. Mix with the scallion and chives.
Blanche tofu, place in a cheese cloth and squeeze, then crumble. Mix with the other vegetables.
To the vegetables, add ground beef, flour, egg yolks, and the seasonings. Mix thoroughly.
Grab a small piece of the filling and pan fry to taste. Adjust with salt and pepper as needed.
When the filling is ready to go, get 1 tablespoon of the filling and place in the center of the dumpling skin. Brush edges with egg yolk to close the dumpling, making sure there is no air in the center and edges are closed tightly.
In a lined baking sheet, place the dumplings with even spaces and freeze for an hour until skins have dried out and are ready to be transferred in zipper bags. Store for 2-3 months.
You can also cook it right away by steaming or frying.
To steam: Line steamers with parchment paper that has been lightly brushed with oil. Steam for 8-10 minutes. Serve with dipping sauce.
To fry: In frying pan with 2-3 tablespoons of oil, let sit for 3-4 minutes per side until golden brown. Or deep fry until golden brown. Serve with dipping sauce.