CUT squash into quarters. Scoop out and discard seeds. Roast pieces in a large baking dish covered with foil, until fork-tender, about 1 hour. Remove from oven. (Reserve some squash to garnish, if desired.) When cool enough to handle, scoop flesh from skins. Whirl in a food processor until smooth. (Purée refrigerates for up to 3 days.)
For pound cake: Turn oven heat down to 350F.
STIR flour with cinnamon, ginger, baking powder and salt in a medium bowl.
BEAT granulated sugar with butter in a medium bowl, using an electric mixer on medium-high, until combined. Beat in eggs, 1 cup butternut squash purée and vanilla.
BEAT flour mixture into squash mixture. Scrape into a lightly oiled 9 5-in. loaf pan. Bake until a skewer inserted into cake comes out clean, 1 hour 5 min to 1 hour 10 min. Cool in pan on a rack for 10 min, then turn loaf out onto rack to cool completely.