Grease and flour 2 loaf pans. Mix together in this order: brown sugar, white sugar, oil, egg, buttermilk, salt, soda, cinnamon, vanilla and flour. Mix in 1 1/2 cups diced rhubarb and chopped nuts. Pour into prepared pans. Top with brown sugar mixed with margarine. Bake at 375 f. for 1 hour. (less for mini pans.)