Boil sweet potatoes whole in their skin for 50 to 55 minutes, or until soft. Run cold water over the sweet potatoes, and remove the skin. Blend the peeled potatoes in a blender and pulse for about one minute to remove all fibers, creating a smooth puree.

Place the sweet potato puree in a large bowl. Add the softened whipped butter and mix well. Using an electric mixer, blend in the light brown sugar, milk, eggs, ground pumpkin pie spice, ground cinnamon, and vanilla extract. Beat on medium speed until the mixture is completely smooth and well combined.

Pour the smooth filling into the unbaked 9-inch pie crust. Bake in a preheated oven at 350 degrees Fahrenheit (175 degrees Celsius) for 55 to 60 minutes, or until a knife inserted into the center comes out clean. Let cool before serving. Serve with cool whip, if desired, and enjoy!