Make the sauce: Stir together all ingredients in a small bowl; set aside.

Make the noodles: Bring a medium pot of water to a boil over medium-high. Add noodles, and cook according to package directions until chewy and al dente. Transfer noodles to a colander, and rinse under cold water; drain well. Spread noodles evenly on a clean kitchen towel, and pat dry. (This helps the noodles crisp when stir-fried.) Set aside.

Heat a large wok or skillet over high for 1 minute. Add canola oil, and swirl to coat wok; heat until oil smokes slightly, 1 to 2 minutes. Add onion; cook, stirring often, until edges are golden brown, about 1 minute. Add ginger and garlic; cook, stirring often, until fragrant and lightly browned, about 20 seconds. Add reserved sauce; cook, stirring often, until slightly reduced, about 15 seconds. Add butter; cook, stirring constantly, until sauce is bubbling, about 15 seconds.

Add reserved noodles to wok; toss well to coat in sauce, separating any noodles that clump together. Spread noodles into an even layer; let sizzle, undisturbed, for 1 minute. Toss well; repeat spreading and sizzling process until edges of noodles are slightly crisped, 2 to 3 more times.

Add bean sprouts and scallions to noodle mixture; cook, stirring often, until just softened, about 30 seconds. Drizzle with chile crisp (if using), and serve immediately.