Bring a large pot of salted water to a boil and cook noodles until just al dente; reserve 1 cup pasta water, then drain.
While noodles cook, heat vegetable oil in a large skillet over medium heat.
Add minced garlic and sauté gently until fragrant and just golden — avoid burning.
Stir in butter and let it melt, allowing garlic to infuse the butter.
Add soy sauce, oyster sauce (if using), dark soy/tamari, and brown sugar; stir to combine.
Pour in 2–4 tablespoons reserved pasta water to loosen the sauce and create a glossy consistency.
Add drained noodles to the skillet and toss thoroughly to coat every strand.
Adjust seasoning with sesame oil, black pepper, and a splash more soy if needed.
If sauce is too thick, add more reserved pasta water; if too thin, simmer briefly to concentrate.
Remove from heat, fold in most of the sliced green onions and sprinkle sesame seeds; plate and garnish with remaining green onions and optional chili oil.