Rhubarb Walnut Muffins

1 1/2 cups flour
3/4 cups brown sugar
1/2 teaspoon baking soda
1/2 teaspoon salt
1/3 cup vegetable oil
1 large egg
1/2 cup buttermilk
1 teaspoon vanilla
1 cup chopped rhubarb
1/2 cup chopped walnuts

Topping:
1/2 cup brown sugar
1/4 cup chopped walnuts
1/2 teaspoon cinnamon

Preheat oven 325 degrees F.
Combine dry ingredients. Add wet ingredients, mixing only until moist. Fold in rhubarb and walnuts. Combine topping ingredients and sprinkle over muffins. Bake for 20 to 25 minutes.