For the gnudi: Thoroughly combine the cheeses, yolks, flour, breadcrumbs and salt in large bowl. Form the mixture into 15-gram (1/2-ounce) balls. Fill a shallow dish with a 1/2-inch of semolina flour. Roll the balls in the semolina flour until they are lightly coated. Put the balls on top of the bed of semolina flour in the dish. Allow the balls to dry in the refrigerator, uncovered, overnight.
For the butter sauce: Combine the cornstarch with 2 tablespoons of the milk in a small bowl. Bring the remaining milk to a boil in a medium pot. Add the cornstarch slurry and whisk together until the milk thickens, about 1 minute. Reduce the heat to medium and slowly whisk in the cubed butter until combined. Remove from the heat and strain the mixture through a fine mesh strainer. Add the truffles, if using, and the salt.
Bring a large pot of salted water to a boil. Heat a saute pan over medium heat and coat the bottom of the pan with olive oil. Add the mushrooms, a pinch of salt and freshly ground black pepper. Then add the peas and a pinch of salt and cook until bright green. Add a little bit of the boiling water to stop the cooking process and remove from the heat.
Add the gnudi to the water. Add some of the butter sauce (about two ladles full) to the mushrooms and peas. Once the gnudi float, remove them with a slotted spoon, draining them well, and add them to the butter sauce. Baste the gnudi with the sauce. Transfer the pasta to serving dishes and top with the Parmesan and more truffles, if using.
Recipe courtesy of Cellaio Steak, Monticello, NY, by Scott Conant