Prepare and cook sweet potatoes...heat enough salted water to cover potates to boiling. Add potatoes. Cover and heat to boiling again. Cook until tender, 30 to 35 minutes; drain. Rinse off in cool water and slip off skins.
Beat cooked sweet potatoes, sour cream, egg yolk, salt and mace in mixer bowl on medium speed until smooth.
Pour into buttered 1 quart casserole; top with marshmallows.
Cook uncovered in 350 degree oven until marshmallows are puffed and golden brown, about 30 minutes. Serves 4...so I usually double.
Hints...can use 1 can (17 ounces) vacuum-packed sweet potatoes as substitute for fresh.
To make earlier in day, put into crock pot and keep on warm. You can either then put into a baking dish and bake until marshmallows are puffed and brown, or just serve with marshmallows on top. Or serve with the nut filling from the Hungarian Nut Roll recipe also posted on the site! Very delicious topping for sweet potatoes!