Place the noodles in a large bowl and add warm water to cover. Let stand, stirring occasionally, for 30 to 40 minutes; the noodles will become pliable but not fully tender. Drain in a colander, then shake to remove excess water. Drizzle with 1 tablespoon oil and toss; set near the stove.

As the noodles soak, in a small bowl, whisk together the soy sauce, Shaoxing wine, sugar, molasses, fish sauce, ¼ teaspoon salt and 1 teaspoon white pepper. Measure 2 tablespoons of the mixture into a medium bowl, stir in the cornstarch, then stir in the beef.

In a 12- to 14-inch wok over high or a 12-inch nonstick skillet over medium-high, heat 1 tablespoon oil until shimmering. Add the beef in an even layer and cook, stirring only once or twice, until well browned and no longer pink, about 5 minutes. Transfer to a plate.

In the now-empty pan, heat the remaining 1 tablespoon oil until shimmering. Add the Broccolini and cook, stirring, until spottily charred, about 2 minutes. Add the garlic and cook, stirring, until fragrant, about 30 seconds. Add the noodles and toss. Pour the soy sauce mixture down the sides of the pan, around the circumference; it should bubble immediately. Cook, tossing and moving the noodles in a circular motion against the sides of the pan, until the liquid is absorbed and the noodles are just shy of tender, about 2 minutes.

Add the scallions and cook, tossing, until beginning to wilt, about 1 minute. Add the beef and accumulated juices; cook, tossing, until the noodles are tender, about 2 minutes. Taste the noodles; if they are still too firm, drizzle in water 2 tablespoons at a time and cook, stirring, until tender. Stir in the vinegar, then taste and season with additional soy sauce and white pepper.