In a large bowl combine flour, brown sugar, baking soda, and salt. Combine oil, egg, buttermilk and vanilla in a small bowl. Stir into the dry ingredients, mix until just moistened. Fold in the rhubarb and nuts. Fill greased muffin tins 2/3 full.
¼ cups packed brown sugar
¼ cup chopped walnuts
½ teaspoon ground cinnamon
Combine the topping ingredients and sprinkle over the muffins.
Bake at 375° for 20-25 minutes. Cool 5 minutes before removing from the pan.