In a large pot, bring 4 quarts of water to a boil.
Meanwhile, in a large saucepan, whisk 1 1/2 cups cold water and cornstarch until smooth. Add the pecorino and stir until evenly moistened. Set the pan over medium-low and cook, whisking constantly, until the cheese melts and the mixture comes to a gentle simmer and thickens slightly, about 5 minutes. Remove from the heat and set aside.
Stir the pasta and salt into the boiling water and cook until al dente. Reserve about ½ cup of the cooking water, then drain the pasta very well. Return the pasta to the pot and let cool for about 1 minute.
Pour the pecorino mixture over the pasta and toss with tongs until combined, then toss in the pepper. Let stand, tossing 2 or 3 times, until most of the liquid has been absorbed, about 3 minutes. The pasta should be creamy but not loose. If needed, toss in reserved pasta water 1 tablespoon at a time to adjust the consistency. Transfer to a warmed serving bowl and serve, passing more pecorino and pepper on the side.
Nutrition information per serving: