Preheat oven to 425°. On a lightly floured surface, roll dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim crust to 1/2 in. beyond rim of plate; flute edge. Refrigerate while preparing filling, Place sweet potatoes in a medium saucepan; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, until tender, 13-15 minutes. Drain potatoes; return to pan. Mash until very smooth; cool to room temperature. , In a bowl, cream butter and sugar. Add eggs; mix well. Add milk, two cups mashed sweet potatoes, vanilla, cinnamon, nutmeg and salt; mix well. Pour into crust. Bake for 15 minutes. Reduce heat to 350°; bake until set or a knife inserted in the center comes out clean, 35-40 minutes. Cool on a wire rack. Refrigerate leftover.