In a medium mixing bowl, combine brown sugar, buttermilk, oil, egg, and vanilla; mix until combined. Set aside.
In a large mixing bowl, combine flour, rhubarb, walnuts, baking powder, baking soda, and salt and mix well. Add wet ingredients to dry ingredients; mix just until combined. Pour evenly into two 8” x 4” greased/floured loaf pans.
For streusel topping, combine sugar, butter, and cinnamon in a small bowl and mix with a fork until it has a wet sand consistency; sprinkle over batter.
Bake at 350° for 60 minutes or until a toothpick inserted in the center comes out clean.
Cool for 10 minutes before removing from pans to wire racks. Cut with a serrated knife.