When cooking the pasta, add a potato to the salted water. This trick returns to the pasta the starch it tends to lose in draining, thus preventing the linguine from clumping. Drain the pasta cubes along with the pasta, reserving a little of the cooking water. Transfer the pasta and potato to a large bowl and dress them with the pesto and butter. If the pasta looks too dry, add a little of the reserved cooking water. Serve in individual dishes, topping each with grated Parmesan cheese and pepper.
If using a food processor, put the garlic, pine nuts, a pinch of salt, the basil, and the oil into the bowl and process it until the ingredients form a smooth paste. Add the Parmesan and the pecorino and pulse just to combine. Taste and adjust the salt.
Recipe courtesy of Benedetta Vitali's SOFFRITO (10 Speed Press, 2001)