1. Preheat oven to 350 F. Grease 2 8x4-inch loaf pans.
  2. Combine in a bowl the brown sugar and oil. Stir well until smooth.
  3. Add the egg, buttermilk, salt, baking soda, vanilla and flour. Blend until moist.
  4. Fold in the diced rhubarb and chopped nuts. Turn batter into prepared loaf pans.
  5. Combine the butter and Sugar until crumbly, then sprinkle over batter.
  6. Bake at 350 F 50 to 55 minutes, or until bread passes the toothpick test. Turn out onto racks and cool before slicing. Slice into about 20 slices per loaf.
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