Bring a large pot of water to a rolling boil. Add 1 tablespoon kosher salt (about 1 1/2 teaspoons per quart). Add the pasta and cook 2 minutes less than the package directions for al dente. 2. While the pasta cooks, set a large skillet over medium-low heat. Add the butter and let it melt slowly — you want it just melted, not bubbling or browning. 3. Add the garlic to the butter. Cook for 45-60 seconds, stirring constantly, until fragrant and just softened. Do not let it brown. 4. Using a liquid measuring cup, scoop out 2 cups of the starchy pasta water. Reserve. 5. Drain the pasta (do not rinse) and add it directly to the skillet with the garlic butter. Toss well to coat. 6. Remove the skillet from the heat. Add the Parmesan cheese in three additions, tossing vigorously after each. Between additions, add a splash of pasta water (about 1/4 cup total per addition) and toss until the sauce looks creamy and coats the pasta. You may not need all the water. 7. Add the salt, black pepper, and red pepper flakes (if using). Toss again. The sauce should look silky and cling to the pasta — if it looks dry, add another tablespoon of pasta water. 8. Serve immediately in warm bowls, topped with extra Parmesan and a crack of black pepper.