Cut the pumpkin into quarters. Scoop out all the seeds and slime from the inner part of the pumpkin and discard. Peel or cut off the outer skin and discard. Slice the pumpkin flesh into 1 inch wedges.
Mix together all the dry ingredients (brown sugar and spices) in a bowl.
Place the pumpkin in a large food safe bag. Sprinkle in the sugar and spice mixture.
Cut butter into 1/2 inch cubes and place in the bag.
Make sure that the pumpkin is one flat layer and not stacked.
Vacuum seal the bag.
Place in water bath and cook for 2-3 hours.
Remove from the water bath. Cut bag and dump pumpkin and the liquid in the bag into a bowl. Puree using a stand mixer, potato masher or hand blender.
Serve as a side dish or use for pie filling (instructions to make pumpkin pie below).
Notes
To make pumpkin pie Mix 2 1/4 cups pumpkin puree with 1 1/4 cups heavy cream and 3 eggs until smooth. Pour into an unbaked pie shell. Bake in a 400F oven for 10 minutes then lower the oven temperature to 350F and bake for an additional 25 minutes or until the sides have puffed up and the middle has very little wiggle when the pie is gently shaken back and forth.