Peel potatoes and cut into cubes. Place in a large stock pot and cover with cold water. Bring to boil and then simmer until fork tender. Drain. Potatoes can be made a day or two ahead and stored covered in the refrigerator.
Preheat oven to 375 degrees F.
Butter a 1 1/2 qt. round baking dish.
With a hand or stand mixer, puree the cooked sweet potatoes together with the cream, butter, egg yolks, salt, pepper and nutmeg.
With a hand or stand mixer (in a clean bowl) beat the egg whites on high until stiff peaks form.
Using a spatula, fold in half the egg whites to the sweet potato mixture. Then fold in the other half until well mixed.
Transfer to the buttered baking dish and smooth the top.
Bake for 50 - 60 minutes until top is browned and the soufflé is firm to the touch.
Squeeze the juice from the lemon half and discard the seeds. Whisk together the fresh lemon juice with the maple syrup. Serve with the soufflé.