Peel the sweet potatoes and cut them into slices.
Melt the butter or margarine in a heavy skillet and add the sliced sweet potatoes.
Mix the sugar, cinnamon, nutmeg and salt. Cover the sweet potatoes with sugar mixture and stir. Cover skillet, reduce heat to low and cook for about 1 hour or until potatoes are candied, stirring occasionally. When potatoes are tender, remove from heat. Stir in the vanilla just before serving. Serve hot.