Scrub the sweet potatoes and poke several holes in them with a fork. Put them in a baking dish or on an oven rack lined with foil. Bake at 425°F for 40 to 50 minutes. When cool enough to handle, scoop 2 cups of flesh from the skins.

Press the pie crust dough into a 9-inch pie pan. Line it with foil and fill with pie weights.

Bake at 425°F for 12 to 15 minutes, until the crust edge just begins to turn golden. Reduce the oven to 350°F, remove the weights and foil, and bake for another 5 minutes. Set the crust aside while making the filling.

Mash or beat the 2 cups of sweet potato flesh with the softened butter. Add the sweetened condensed milk, beaten eggs, vanilla, cinnamon, and nutmeg; mix well.

Pour the filling into the pie crust and cover the crust edges with a pie shield or foil. Bake at 350°F for 40 to 50 minutes. The center should jiggle just a little, and a knife inserted at the edge of the pie should come out clean.

Cool at room temperature, refrigerating the pie within 2 hours. Chill completely in the refrigerator before serving, with whipped cream if desired. If reheating a leftover slice, use a food thermometer to check that it reaches 165°F.