1. Wash hands with soap and water. Peel sweet potatoes and chop roughly into about 2-inch pieces. Place in a large pan. Cover with cold water, bring to a boil and boil uncovered until potatoes are tender, about 30 minutes. Drain and set aside.
2. Preheat oven to 375 degrees F for shallow pies. Or 425°F for deep-dish version. Position the rack so it is not on the bottom position but second to the bottom position.
3. In a large bowl, combine brown sugar, sugar substitute, salt, nutmeg, cinnamon, egg and egg whites, milk and vanilla. Beat with an electric mixer until smooth, about 30 seconds, on high speed.
4. Mash sweet potatoes with a fork or potato masher. Measure 3 cups and add to egg mixture. Beat until smooth.
5. Divide evenly between 2 regular thawed pie shells, or pour into one 9-inch deep-dish pie shell.
6. Bake shallow pies 25-30 minutes. Bake the deep-dish pie for 15 minutes at 425°F, then reduce heat to 350°F for 40 to 50 minutes or until the knife inserted in the middle comes out clean. Cool pie(s) about 2 hours before slicing. Serve immediately or chill. 

For dessert, top with two tablespoons of fat-free whipped topping if desired. Cut shallow pie(s) into 8 servings or deep-dish pie into 16 slices.

Store covered in the refrigerator for up to four days.