Melt the butter 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". They should be tender but a little hard around the edges. Also the sauce will turn dark. You will need to stir occasionally during the cooking. Stir in the vanilla just before serving. Serve hot.