Preheat oven to 350 degrees. Prepare two loaf pans with baking spray.
In a large mixing bowl, combine the egg, oil, buttermilk, and vanilla.
Add brown sugar, flour, salt, and baking soda, and stir to combine. Don't over-mix.
Next, add the diced rhubarb and pecans (optional), and gently stir in.
Pour batter evenly into two prepared loaf pans.
Mix the streusel topping by combining the sugar and butter until crumbly. Sprinkle over both loafs.
Bake for approximately 40 - 45 minutes, or until a toothpick, when inserted, comes out clean.