In a small bowl, mix together light soy sauce, dark soy sauce, sugar, and white pepper (if using) until sugar dissolves. Set aside.
Heat the oil in a wok or large skillet over medium-low heat. Add the scallion pieces and cook, stirring occasionally, until they turn dark brown and crispy, about 10-12 minutes. Do not burn.
Remove the crispy scallions with a slotted spoon and set aside. Leave the oil in the pan.
Increase heat to medium-high. Carefully pour the soy sauce mixture into the hot oil (it will sizzle). Stir and cook for 30 seconds, then turn off heat.
Meanwhile, cook the noodles according to package directions until al dente. Drain well.
Add the drained noodles to the wok with the scallion oil and toss to coat evenly.
Serve immediately, topped with the crispy scallions.
Tips
For extra flavor, add a teaspoon of toasted sesame oil at the end.
If you prefer a thicker sauce, mix 1 teaspoon cornstarch with 2 tablespoons water and add with the soy sauce mixture.
Leftover scallion oil can be stored in the refrigerator for up to 2 weeks.