Preheat oven to 350 °F and line 24 muffin cups with liners.
In a large bowl, stir together sugar, brown sugar, oil, buttermilk, egg, and vanilla.
Add flour and baking soda, stirring until just combined.
Fold in diced rhubarb and chopped nuts.
In a small bowl, mix together sugar, cinnamon, and butter with a fork until combined.
Fill each muffin cup ⅔ full with batter.
Top each muffin with about 1 teaspoon of the cinnamon sugar topping.
Bake for 18-22 minutes. (Mine took 32 min.)
Remove from the muffin tin and cool on a wire rack.
