Preheat oven to 375°F (190°C). Grease or line a 12-cup muffin tin.
In a large bowl, whisk together flours, baking soda, baking powder, salt, and cinnamon.
In a separate bowl, beat together brown sugar, oil, egg, buttermilk, and vanilla until smooth.
Add the wet ingredients to the dry and stir until just combined. Fold in chopped rhubarb.
Spoon batter evenly into muffin cups.
For topping (optional), mix ingredients together and sprinkle over batter.
Bake 20–25 minutes or until a toothpick inserted comes out clean.
Cool in the pan for 5 minutes, then transfer to a wire rack.