Melt butter in a heavy skillet and add sliced sweet potatoes.
Mix sugar, cinnamon, and nutmeg. Cover 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 sauce will turn dark. You will need to stir occasionally during cooking. Stir in vanilla just before serving. Serve hot.