Bring a large pot of generously salted water to a boil. Cook the spaghetti according to the package directions until al dente, keeping the noodles tender but still slightly firm in the center.

Before draining, scoop out about 1/2 cup of the pasta cooking water and set it aside. Drain the spaghetti well, but do not rinse it because the surface starch helps the sauce cling to the noodles.

While the pasta cooks, melt the butter in a large skillet over medium-low heat. Add the minced garlic and cook for 30–60 seconds, just until fragrant; do not let it brown or burn.

Add the drained spaghetti to the skillet and toss until each strand is coated in the garlic butter. If using olive oil or red pepper flakes, add them at this stage and toss to distribute.

Take the skillet off the heat or reduce it to very low, then add the Parmesan, black pepper, and parsley. Toss thoroughly while adding reserved pasta water one or two tablespoons at a time until the sauce looks glossy and lightly coats the noodles.

Taste the spaghetti and adjust the salt as needed. Serve immediately with extra Parmesan and parsley, adding a final twist of black pepper if desired.