Boil sweet potatoes whole in skin for 40 to 50 minutes, or until done. Run cold water over the sweet potato, and remove the skin. Break apart potatoes in a bowl.
Add butter,and mix well with mixer. Stir in sugar, milk, eggs, nutmeg, cinnamon, and vanilla. Beat on medium speed until mixture is smooth. Pour filing into an unbaked pie crust. Bake at 350 for 55 to 60 minutes or until knife inserted in center comes out clean.
Pie will puff up like a souffle, and then will sink down as it cools.