Preheat oven to 350° F.
Line muffin pans for 18 muffins with paper cups or grease.
In medium bowl, stir together flour, baking soda, baking powder, salt, and 1 teaspoon cinnamon.
In bowl of stand mixer add 1 cup sugar, oil, egg, vanilla, and buttermilk; beat until smooth.
Pour in dry ingredients and mix until just blended. Stir in rhubarb and walnuts.
Spoon batter into prepared muffin cups, filling each 3/4.
For topping, combine melted butter, remaining 1/3 cup sugar and 1 teaspoon cinnamon; sprinkle on top of muffins.
Bake about 25 minutes, or until toothpick inserted in center comes out clean.
Allow muffins to cool in 5 minutes in pans; remove to wire rack to cool. Best when served warm.