Preheat oven to 350 degrees. Grease two 12 cup muffin pans or line with paper cups.
2
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 just until blended.
3
Stir in 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 1teaspoon of the mixture on top of each muffin.
4
Bake in preheated oven until the tops of the muffins spring back when lightly pressed, about 25min. Cool in the pans for a least 10min before removing.
