Brown the butter: Place the butter in a sauté pan over a high heat. Let it foam, then watch as it turns from golden to brown. Don't take it off the heat until you smell the sweet, nutty aroma. This takes about 3 minutes.
Add the garlic: Remove the pan from the heat. Stir in the garlic and sage leaves if using. The residual heat cooks the garlic without burning it.
Add the pasta water: Pour in a ladleful of the reserved starchy pasta water. Return the pan to the heat and stir for 1 to 2 minutes.
Finish with Parmesan: Add the cooked pasta directly to the pan with the Parmesan. Toss until the cheese melts and the sauce coats every strand. Add a little more pasta water if it needs loosening. Season with black pepper.
Serve immediately: Alfredo sauce thickens as it cools. Get it to the bowl while it's still glossy.
