Preheat the oven to 400°F and line a muffin tin with baking cups or lightly grease it with butter.
In a medium bowl, add flour, baking soda, baking powder, cinnamon, and salt and stir to combine.
In a large bowl, add brown sugar and melted butter and whisk until well combined. Add egg, vanilla extract, and buttermilk and whisk until smooth.
Transfer the dry ingredients to the wet ingredients and fold gently to combine until well incorporated. Add chopped rhubarb to the batter and fold again, be careful not to over-mix the batter.
Divide the batter evenly across the muffin liners and then transfer to the oven to bake for 20-22 minutes until lightly golden and/or a toothpick comes out clean.
Once baked, remove from the oven and allow to cool completely to room temperature in the muffin pan or for at least 10 minutes before removing from the muffin tins. Once removed, transfer the muffins to racks to cool completely.
Optional: Once cooled, add the sugar topping. In a small bowl, add white sugar and brown sugar and stir to combine. Brush the top of each muffin with a bit of melted butter and sprinkle with the sugar mixture.
The muffins can be served immediately or stored in an airtight container for 5 days or in the freezer for 3 months.