Whisk the grated pecorino, black pepper, and potato starch together until the cheese is evenly coated in the starch.
Bring the water and salt to a boil over high heat and add the pasta.
When the pasta is a little over a minute from being done, add a ladleful of the boiling liquid to the grated cheese mixture in a pan. Use a spatula to mix it together with the cheese to form a smooth sauce. Add more liquid if needed.
Then use a slotted spoon to transfer the pasta to the sauce. Turn on the heat and mix the pasta and sauce until it's thick enough to coat the pasta. If the pasta starts getting sticky, just add some more pasta liquid.