Cook the noodles using the package directions, then drain and set aside.
Heat vegetable oil in a large skillet or wok over medium-high.
Add the shiitake mushrooms and sauté about 3 minutes until they start to soften and brown.
Add garlic and stir for 30 seconds until fragrant—try not to let it burn.
Lower the heat to medium and add butter, letting it melt all the way.
Add the cooked noodles and toss so they're coated in butter and aromatics.
Pour in chicken stock, stir fry sauce, soy sauce, sesame oil, and sugar.
Toss everything to coat the noodles and mushrooms evenly.
Cook for another 2-3 minutes, stirring often, until the sauce thickens up a bit.
Sprinkle in black pepper and most of the green onions (save a little for garnish) and toss again.
Serve hot, topped with the rest of the green onions.