Butternut squash is halved and roasted with spices (nutmeg, cinnamon, pepper and star anise) and butter, then puréed. Of course there will be more butter in the cake itself, but it is browned first. From that point on, Mr. Allegretti conceded, “it’s pretty much a classic pound cake.” The plate is filled out with a squash and pumpkin ice cream and a pine-nut brittle.