Sweet Potato Muffins

1 1/2 cups sifted all-purpose flour
1/2 cup sugar
1/2 cup sweet potatoes, cooked, mashed
1/2 cup walnuts, chopped
1/4 cup butter, melted
1/2 cup milk
2 teaspoons baking powder
1 teaspoon ground cinnamon
1/2 teaspoon salt
1/2 teaspoon ground nutmeg
1 egg
1 tablespoon sugar