Add butter to warm squash puree (or melt butter first) in a large bowl. Blend in brown sugar and eliminate any chunks that may form. Add the remaining ingredients and whisk until everything is mixed together.
Pour filling into an unbaked 9" pie shell. Bake for about 50 - 60 minutes in a 350 degree oven. The crust will be a nice golden brown and the center should be set.
Let the pie cool (if you can) and serve.
*Add a little brown sugar and pumpkin pie spice to any leftover butternut squash puree for a nice side dish or snack.*My butternut squash was just over 3 lbs. and I had a cup or so left over puree.