Preheat the oven to 350° F.
Wash the outside of your pumpkin, turn it onto its side, and cut it in half. If the pumpkin is being stubborn, you can gently tap the knife with a rubber mallet to get it cut open. Try to cut it as evenly in half as possible. Leave the stem on; when it gets soft later, the stem will easily pop off.
Using a metal spoon, scrape out all the guts, making sure to remove most if not all of the strings. You can separate the seeds and save them to plant the next year or turn them into roasted pumpkin seeds.
Grease one (or two–depends on the size of the pumpkin) glass baking dish(es) with butter. It is best to use a glass dish because often the metal ones get too hot and can burn the pumpkin.
Place the pumpkin halves in the dish(es), cut side down.
Fill the dish halfway with cold water.
Put the halves in the preheated oven for up to 1-1/2 hour (90 minutes) or until the flesh is the consistency of room temperature butter. The skin will be wrinkly and a burnt orange color and the pumpkin half may sink in a bit.
Pull off the stem. It should come loose with little effort.
Using a spatula and an oven mitt, carefully remove one half of the pumpkin.
Using a spoon and caution as it'll be very hot, start scooping out the flesh. It’ll be very soft and very easy to remove. It’s easier to remove and puree the pulp when it is hot versus after it’s cooled.
Start scooping the puree into a food processor. Now is a good time to add any spices or other ingredients that you want. If you don’t have a food processor, mash it with a potato masher or puree with a blender.
Thoroughly puree the pumpkin then set it aside to cool.
Once the puree has cooled, scoop it into a labeled freezer safe jar with an airtight lid. Refrigerate and use within 4 days or freeze for up to 6 months.
Remember to mark your containers with its contents (pumpkin puree, spices, etc.) and the date you prepared it.