Preheat oven to 400 degrees F.
Brush flesh sides of pumpkin wedges with butter. Arrange, flesh side down, in a large roasting pan, and cover pan tightly with foil. Bake, turning pieces after 1 hour, until flesh is very tender when pierced with a fork, about 2 hours. Remove foil and cool pumpkin in pan until it can be handled.
Discard any liquid that may have accumulated and scoop out pumpkin flesh, then puree pulp in batches in a food processor. Transfer to a large sieve or colander lined with several layers of cheesecloth set over a large bowl.
Drain puree, chilled, its surface covered with plastic wrap, 8 hours or overnight.
Puree keeps, covered and chilled, 2 days or, frozen in an airtight container, 2 months.