Preheat the oven to 350° F.
Grease two 12-cup muffin pans or line them with cupcake wrappers.
In a medium bowl, stir together the flour, baking soda, baking powder, and salt.
In a separate bowl, beat the brown sugar, oil, egg, vanilla, and buttermilk with an electric mixer until smooth.
Pour in the dry ingredients and mix by hand until blended. Stir in the rhubarb and walnuts.
Spoon the batter into the prepared cups, filling almost to the top.
In a small bowl, stir together the melted butter, white sugar and cinnamon.
Sprinkle about a teaspoon of this mixture on top of each muffin.
Bake until the tops of the muffins spring back when lightly pressed, about 25 minutes.
Cool for at least 10 minutes before removing from pans.