Cook noodles according to package instructions until al dente. Drain and set aside.
In a bowl, whisk together soy sauce, water, brown sugar, garlic, ginger, and cornstarch until smooth.
In a large skillet over medium-high heat, cook ground beef until browned, breaking it apart as it cooks. Drain excess fat if needed.
Reduce heat to medium. Pour sauce into the skillet and stir continuously until thick and glossy.
Add cooked noodles to the skillet and toss until evenly coated.
Cook for 1–2 minutes to allow noodles to absorb the sauce.
Remove from heat and garnish with green onions or sesame seeds before serving.