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. 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 1 teaspoon of this mixture on top of each muffin.