Toasted pecans make a delicious garnish, adding crunch and adorning it like a necklace.
Preheat oven to 400°.
Wrap sweet potato in foil; bake at 400° for 45 minutes or until tender. Cool completely. Peel and coarsely chop sweet potato.
Fit dough into a 9-inch pie plate coated with baking spray. Press dough against bottom and sides of pan. Fold edges under, and flute decoratively. Line bottom of dough with a piece of foil; arrange pie weights or dried beans on foil. Bake at 400° for 15 minutes. Remove pie weights and foil; cool crust on wire rack 5 minutes.
Reduce oven temperature to 350°.
Place sweet potato, milk, and next 5 ingredients (through whole eggs) in a food processor. Add 2 egg whites; process until smooth. Add butter; process until combined. Pour mixture into crust. Sprinkle nuts around edge of filling. Combine remaining egg white and 1 teaspoon water in a small bowl, stirring well with a whisk. Brush egg mixture on edges of crust. Bake at 350° for 45 minutes or until almost set in the center, shielding edges of piecrust with foil, if necessary. Cool completely on a wire rack before slicing.