The heat from the noodles and some of the starchy cooking water melts the butter, creating a silky coating. If using a short pasta, start by adding only 2 tablespoons cooking water when tossing the noodles with the butter mixture. Meanwhile, in a large bowl, using a silicone spatula, mash and stir the butter, miso, garlic, lemon juice, the scallion whites and . Reserve about ó cup of the cooking water, then drain. Toss vigorously, adding more cooking water, 1 tablespoon at a time if needed, until lightly sauced; there should not be any liquid pooled in the bowl.