Preheat oven to 400°F. Remove pumpkin stem. Cut pumpkin in half from top to bottom. Scoop out seeds and discard (or save to make your own roasted pumpkin seeds). Cut each piece in half again, so you have 4 quarters. Sprinkle flesh with salt. Place cut-side down on large parchment-lined shallow baking pan.
Roast about 40 minutes, or until skin is soft and flesh is very tender when pierced with a fork. Cool completely on wire rack.
Scoop flesh from skin and transfer to food processor. Process about 3 to 5 minutes, gradually adding water and scraping sides as needed, until smooth. Add cinnamon; pulse until well blended.
Use pumpkin puree for baking, just as you would canned pumpkin. Store in airtight container in refrigerator up to 1 week.