Preheat the oven to 325°F (160°C). Grease a 9×5‑inch loaf pan and In a large bowl whisk together the brown sugar, vegetable oil, egg, and vanilla until smooth.
In another bowl whisk together the flour, baking soda, salt, and cinnamon (if using).
Add the dry ingredients to the wet mixture alternately with the buttermilk, stirring gently until just combined.
Fold in the chopped rhubarb, and add nuts if using, mixing until evenly distributed.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
In a small bowl, combine brown sugar, cinnamon, and melted butter for the topping; sprinkle evenly over the batter.
Bake for 60–70 minutes.
Until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for about 15 minutes, then transfer to a cooling rack to cool completely before slicing.