Heat oven to 400° F. Pierce each sweet potato several times with the tines of a fork. Place the sweet potatoes on a rimmed baking sheet lined with foil. Bake until tender, about 45 minutes.
Cool. Scrape the pulp out of the skin, transfer to a large bowl, and mash. Reserve
In a mixing bowl, cream butter and sugar.
Add eggs, one at a time; mix well.
Add milk, sweet potatoes, vanilla, cinnamon, nutmeg, and salt; mix until very smooth.
Pour into pie shell.
Bake at 425 degrees for 15 minutes.
Reduce heat to 350 degrees; bake 35-40 minutes longer or until pie tests done.