Heat oven to 400 degrees. In a large bowl, cream the butter with the sugar, salt, nutmeg, and cinnamon until creamy. Add eggs, one at a time beating well, with mixer. At low speed, add mashed potatoes, alternating with milk until fluffy. Pour mixture into pie shell. Bake for 50 minutes or until knife inserted comes out clean. Cool and serve. This amazing southern sweet potato pie will become the main dessert feature at these festive meals or any other special day. My grandmother would be honored if you tried this great pie. She was one of the first creator of this special pie. Served with any meal, this pie will keep the smiles on the faces of all family members, guests, and friends. I assure you that you will be surprised at all the compliments you will receive.