Boil sweet potato whole in skin 40-50 minutes. Put in bowl of cold water and remove skin. Break apart sweet potato in bowl. Add butter and mix well. Stir in sugar, milk, eggs, nutmeg, cinnamon and vanilla. Beat until smooth. Pour into unbaked pie crust. Bake at 350 degrees for an hour or more, until a knife comes out clean. The pie will puff up, then sink as it cools.