Place cubed sweet potatoes in a medium saucepan and add water to cover. Bring to a boil, reduce heat to medium and cook 15 minutes, uncovered, until tender.
Preheat oven to 425 degrees F. Prepare a single pie crust in a pie pan and set aside.
Drain potatoes and return to pan. Mash until smooth, and allow to cool to room temperature.
In a large mixing bowl, cream butter and sugar together until smooth. Add eggs and mix well. Add (2c) mashed sweet potatoes, milk, vanilla, nutmeg, cinnamon and salt. Mix on medium speed until incorporated and smooth. Pour filling into pie crust.
Bake for 15 minutes, then reduce oven temperature to 350 degrees F. Continue baking for 30 to 40 minutes, until a knife inserted in center comes out clean. Cool completely and store in refrigerator until ready to eat.
*Nutritional information is an estimate. Please consult a doctor or nutritionist for the most accurate nutritional info.