Preheat oven to 325° F.
In a large bowl, combine 1 1/2 cups brown sugar, 2/3 cup vegetable oil, 1 egg, and 1 teaspoon vanilla extract. Mix well.
In a separate bowl, combine 3 cups all purpose flour, 1 teaspoon salt, and 1 teaspoon baking soda. Mix well.
Stir flour mixture into sugar mixture, alternately adding in 1 cup buttermilk, until all ingredients are fully combined.
Fold in 2 cups chopped rhubarb.
Pour batter into two greased bread pans.
In a small bowl, combine 1/4 cup brown sugar, 1/2 teaspoon cinnamon and 1 Tablespoon melted butter. Sprinkle lightly over the batter in the pans.
Bake at 325° for 60 minutes or until toothpick inserted in center comes out clean.
Let cool in pans at least 30 minutes before removing, then transfer to a wire rack to cool completely.