In a small bowl, add 1 Tablespoon water with pumpkin pie spice until well combined. Allow to rest for 5-10 minutes.
In a medium saucepan, whisk together sweetened condensed milk, brown sugar, water, and pumpkin.
Over medium heat, bring to a boil. Once boiling, reduce heat to a simmer. Add the pumpkin pie spice mixture and whisk to combine. Simmer for 2-3 minutes until smooth and well combined.
Cool slightly and transfer to a mason jar or other container. Refrigerate up to 3 weeks.
Notes
Once the mixture boils, reduce the heat to avoid boiling over.
For a vegan version, use sweetened coconut milk instead of condensed milk
Pour into a glass measuring cup from the pot, then into the mouth of the mason jar to avoid spilling.
Leftovers will keep in an airtight container in the refrigerator for 3 weeks.