Prepare the pumpkin puree. Preheat an oven to 350 degrees F. With a large knife, cut the pumpkins in half, pole-to-pole. Use a large spoon to scoop out the seeds and membrane in the center. (The seeds can be reserved and roasted for a snack.) Rub the exposed edges of the pumpkin with a little olive oil. Place the halved pumpkins cut-side down onto a cookie sheet lined with parchment or a non-stick liner. Roast until completely soft, about an hour. Remove from the oven and allow to cool, then scoop out the softened pumpkin flesh. Two medium sugar pumpkins should yield about 4 cups of puree.
Make the butter. In a medium nonreactive saucepan, combine the pumpkin puree, maple syrup, spices, salt and vinegar. Bring to a simmer, and cook on medium-low heat, stirring frequently to prevent scorching or sticking, until the mixture thickens and darkens, about 30 minutes. If a smoother texture is desired, use an immersion blender to break down the puree further. Remove from heat. Allow to cool, and store in the refrigerator or freezer.