Scrub the outside of the pumpkin, cut it in half and scrape out the seeds and any stringy fiber. Use a small paring knife or vegetable peeler to remove the outer skin, and cut the pumpkin into two-inch chunks. Place the chunks in a steamer basket set in a saucepan, adding just enough water to come up to the bottom of the steamer. Cover and bring to a simmer, and cook until the pumpkin is very (about 20 minutes). Drain the water and puree the pumpkin until completely smooth in a food processor or blender.