Add the macaroni to a large pot and cover completely with the water. Season the water lightly with the salt. Place the pot on the stove over high heat and bring to a boil, stirring often to prevent the pasta from sticking to the bottom of the pot. As the pasta cooks, continue to add water to keep the pasta just barely submerged.
When the pasta is just shy of al dente, stop adding water and let the pasta water cook down slightly as the pasta reaches al dente. The water should be cloudy and thickened from the starch coming out of the macaroni.
Dissolve the cornstarch in the milk and stir the mixture into the pot of pasta. Bring to a simmer, stirring constantly so the pot doesn’t scald on the bottom. When the sauce begins to boil and has thickened, remove the pot from the heat and add the Truffle à la Crème, the butter, and half the shredded cheese. Stir to incorporate the cheese into the sauce. Once the first half of cheese has been melted in, add the second half.
Season the pasta with salt, if needed, and freshly cracked black pepper. Add the chives and transfer to serving dishes. Generously shave the black truffles overtop, if using. Serve immediately.
