Preheat your oven to 350°F and grease your muffin pan(s) with butter or cooking spray. While the oven preheats, chop the rhubarb into 1/2-inch pieces and coarsely chop the walnuts. Set both aside—keeping the rhubarb separate prevents it from releasing too much liquid into the batter before baking.

In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. Whisking aerates the flour and ensures the leavening agents are evenly distributed throughout, which gives you muffins with consistent rise and a light crumb.

In a separate bowl, whisk together the brown sugar, buttermilk, vegetable oil, egg, and vanilla until well combined and smooth. The buttermilk reacts with the baking soda to create lift, so make sure everything is fully incorporated before moving to the next step.

Pour the wet ingredient mixture from Step 3 into the bowl with the dry ingredients from Step 2 and stir gently until just combined—don't overmix, as this can lead to tough muffins. Once the batter just comes together, fold in the chopped rhubarb and walnuts. I like to add the rhubarb at this stage rather than earlier because it stays fresher and releases its tart flavor more effectively when it mixes directly into the batter.

While preparing the topping, divide the batter evenly among the muffin cups, filling each about two-thirds full. In a small bowl, combine the sugar, butter (softened or melted), and cinnamon until it forms a crumbly mixture. Sprinkle the cinnamon sugar topping generously over each muffin, pressing it gently so it adheres to the batter.

Bake in the preheated 350°F oven for 25 minutes, until the tops are golden brown and a toothpick inserted into the center of a muffin comes out with just a few moist crumbs (not completely clean, as the rhubarb will still be slightly moist). Remove from the oven and let the muffins cool in the pan for 10 minutes before turning them out. This resting period allows the structure to set just enough so they release cleanly from the pan without breaking apart.