In a large bowl, combine flour, cornmeal, sugar, baking powder, salt and nutmeg. Cut in butter with two forks, a pastry cutter or your fingers until mixture is crumbly. In a medium bowl, beat egg until frothy. Stir in buttermilk and sweet potatoes. Pour mixture into flour mixture, stirring just until blended. Pour batter into greased 9-inch-square baking dish. Bake in preheated oven 20 minutes or until center springs back when pressed. Cool in pan on wire rack. Cut into squares and serve.