Cook pasta in a large pot of salted boiling water until just shy of al dente. Scoop out 1/2 to 3/4 cup of starchy pasta water before draining and set aside.
Return the empty pot to low heat, add butter and optional minced garlic. Warm until butter is melted and garlic is fragrant but not browned.
Add a small ladle of reserved pasta water to the butter and swirl. Add drained pasta, sprinkle in half the Parmesan, and toss vigorously while adding more pasta water a little at a time until a glossy sauce forms.
Turn off heat, season with salt and lots of freshly ground black pepper, garnish with parsley and extra Parmesan, and serve immediately.
Notes
Use freshly grated Parmesan for the smoothest sauce.
Add pasta water gradually to control sauce consistency.
If sauce breaks or becomes grainy, remove from heat and whisk in warm pasta water slowly.