Bring a large pot of generously salted water to a rolling boil. Add pasta and cook 1 minute less than package directions for al dente.

While pasta cooks, place a 12-inch skillet over medium-low heat. Add butter and let it melt slowly — it should foam gently, not brown.

Add sliced garlic and red pepper flakes to the butter. Cook, stirring constantly, for 60-90 seconds until garlic is fragrant and just pale gold. Watch closely: burnt garlic turns bitter.

Ladle 1/2 cup pasta water into the skillet. Increase heat to medium and let it simmer for 30 seconds, swirling to emulsify the butter into the water.

Use tongs to transfer the cooked pasta directly from the pot into the skillet — no need to drain it fully; a little clinging water is fine.

Toss pasta in the sauce for 1 minute over medium heat, adding more pasta water a tablespoon at a time if it looks dry. The sauce should look creamy and coat each strand, not pool at the bottom.

Remove from heat. Add Parmesan and toss vigorously — the cheese melts into the sauce, thickening it further. Taste and add salt if needed.

Serve immediately, topped with extra Parmesan and parsley if desired.