In large bowl, cream together butter or margarine and brown sugar. Beat in egg and vanilla.
Sift or stir together flour, baking soda and salt; gradually stir in to butter mixture. Fold in sour cream and rhubarb. Spoon batter into greased 9 x 13 inch glass baking dish.
Stir together white sugar and nutmeg; sprinkle over batter.
Bake at 350 degrees F (175 degrees C) for 40 minutes, or until tester comes out clean.