Preheat the oven to 180 degrees C. Grease two 12 cup muffin trays or line with paper muffin cases.
Muffin:In a medium bowl stir together the flour, bicarbonate of 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 mixture into the prepared muffin trays filling almost to the top.
Topping: 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.
Bake in the preheated oven for 25 minutes or until the tops of the muffins spring back when lightly pressed.
Cool in the trays for at least 10 minutes before removing.