Preheat the oven to 375°.

In a medium bowl, whisk together the all-purpose flour, whole wheat flour, baking soda, baking powder, and salt.

In a large bowl, combine the buttermilk (or yogurt), brown sugar, cooking oil, beaten egg, and vanilla.

Fold in the rhubarb.

Fill greased muffin cups ⅔ full.

Mix together the sugar, melted butter, cinnamon, and flour.

Sprinkle on top of the muffins.

Bake for 20 minutes or until done.

Remove the muffins from the pan and cool on a wire rack.