CUT squash into quarters. Scoop out and discard seeds. Roast pieces at in a large baking dish covered with foil, until fork-tender, about 1 hour. Remove from oven. When cool enough to handle, scoop flesh from skins. Whirl in a food processor until smooth. (Refrigerates for up to 3 days.)
COMBINE almond milk with ice, 1/2 cup butternut squash purée, banana, brown sugar, cinnamon, ground ginger and vanilla in a blender. Purée until smooth. Sprinkle with allspice, if desired.