Fill a large pasta pot ⅔ full with hot water. Place the pot on your stove over high heat. Add 1 ½ teaspoons salt and cover the pot.
Grate your Pecorino, if needed. (I grate chunks of the cheese in my blender.)
Add your grated cheese and the black pepper to a stainess steel or glass bowl large enough to fit your cooked pasta. Stir the cheese and pepper to combine.
When the water comes to a boil, add your pasta to the pot, stirring it in. Stir your pasta every few minutes to prevent sticking.
After your pasta has been cooking for at least five minutes, scoop out ½ cup of the cooking water (a standard ladle) and slowly, gradually add it to your cheese mixture, while whisking to combine. This is important for the right consistency. Scoop ¼ cup more (half a ladle) and slowly add it to the cheese while stirring. You want the mixture to be creamy, not watery.
Place an extra cup of cooking water in a separate, small bowl just in case you need it a little later.
When your pasta is cooked (see the al dente directions on your package and taste the pasta a little before that to see if it's done), drain your pasta in a colander in the sink.
Transfer the cooked pasta to your large bowl with the Pecorino and pepper sauce. (This is important. Do not mix everything in your cooking pot, because the cheese will become clumpy.) Toss the pasta in the Cacio e Pepe sauce in the bowl until the spaghetti is coated well. If the pasta seems dry, add a bit more of the reserved water.
Serve the pasta immediately, garnished with parsley, if desired, and with extra pepper and cheese at the table.
If you have any leftovers, add extra reserved water to them to prevent them from being too dry. (See notes.) Store leftovers in the refrigerator for up to four days.
It's important to use a bowl to combine the pasta with the cheese sauce. Don't mix everything in your pot on the stove, because there will be too much heat, and it will cause the Pecorino Romano to clump and harden into huge gobs. Use a stainless steel or glass bowl, because it will retain some heat that's needed to help make a smoother sauce.
This dish is best eaten immediately, or it will start to get dry. If there is any delay in serving anyone, you can stir in a little extra reserved cooking water to that portion. The leftovers will be dry, so you can add some of the pasta water before storing them, but you still might need to add butter or oil when warming them up later.