Cook the sweet potatoes in boiling water until tender. Drain, peel, and mash until smooth.
Add the spices, salt, sugar, butter and cream. Beat together. Add the egg yolks. Beat the egg whites until stiff and fold them into the mixture.
Bake in a buttered soufflé mold or casserole for about thirty-five minutes, or until soufflé is well risen and brown. If desired, brush the top of the soufflé with melted butter and sprinkle lightly with light brown sugar before baking.