Place sweet potatoes into a large pot of boiling water; cook until tender, about 20 minutes.
Turn oven on at 350°F to preheat.
Drain potatoes and cool slightly, then use a spoon to remove potato from the skins. Use potato masher to mash potatoes in a large bowl until smooth and creamy; set aside. (Alternatively, you can use an electric mixer to mash the potatoes, but be very careful, as mixing too long will cause them to become pasty and sticky.)
Cream together the butter and sugar until light and fluffy; Add the eggs and mix until well combined. Add the mashed sweet potatoes, evaporated milk, vanilla, cinnamon, nutmeg and salt. Mix until well combined.
Transfer mixture into unbaked pie shell. and place onto center rack of the preheated oven.
Bake for 50-55 minutes, or until a toothpick inserted in the center of the pie comes out clean. (Tip: Check crust after about 40 minutes. If it's browning too quickly, cover crust with a thin strip of aluminum foil.)
Remove pan from oven and transfer to a wire rack to cool. Serve chilled or at room temperature, using whipped cream as garnish, if desired.