Stir together all the ingredients in a large soup pot. The pumpkin will bubble up, so use at least a 4-quart pot.
Place over medium-high heat and bring to boil. Once boiling, reduce heat to a simmer and simmer for 25 minutes, stirring occasionally, until pumpkin butter has reduced in volume and thickened.
Let cool for 15 minutes before transferring the pumpkin butter to a mason jar. Continue to cool for 1 hour at room temperature. Store in the refrigerator for up to 2 weeks or freeze it for up to 2 months.
Serving and Storing - Store the recipe in an airtight container in the refrigerator for up to 2 weeks or freeze it for up to 2 months.