Cook the noodles: Bring a large pot of salted water to a boil. Cook 8 ounces spaghetti according to package directions until al dente. Reserve ½ cup of pasta water, then drain.
Make the garlic butter: In a large skillet over medium-low heat, melt 4 tablespoons butter. Add 4 cloves minced garlic and cook for 1-2 minutes until fragrant, stirring often to avoid burning.
Combine: Add the drained noodles to the skillet with the garlic butter. Toss to coat. Sprinkle in ½ teaspoon salt, ¼ teaspoon black pepper and ¼ teaspoon red pepper flakes if using.
Finish: Stir in parmesan cheese and parsley. Add a splash of the reserved ½ cup pasta cooking water if the noodles seem dry. Toss until evenly coated. Serve immediately with ¼ cup grated parmesan cheese on top. Garnish with 2 tablespoons chopped fresh parsley if you fancy it.