Meanwhile, heat 1 tablespoon of the butter in large sauté pan over medium heat; add scallions and a pinch of salt and cook until soft, 3 minutes. Add 1/4 cup reserved pasta cooking liquid—dip a measuring cup into the pot if the pasta is still boiling—and all but 1/4 cup corn; simmer until corn is heated through and almost tender, 3 to 5 minutes. Season with salt and pepper to taste, transfer to a blender (or food processor), and purée mixture until smooth, adding a little extra water if needed to get a thick but pourable texture.