Bake sweet potatoes until well done. Scoop potato from skins into mixing bowl. Mix with sugar and butter. Season with spices and salt. Beat until smooth. Beat egg yolk slightly and add to with cream to potato mixture. Blend well. Fold in egg whites, beaten until stiff but not dry, and pour into unbaked pie shell. Bake in 350 degree oven for one hour or until center is set when tested. To brown crust raise oven temperature to 230ºC or last 5 minutes of baking time.