In a large mixing bowl, combine flour, brown sugar, baking soda and salt. Combi ne oil, egg, buttermilk, and vanilla; stir into dry ingredients just until mois tened. Fold in rhubarb and walnuts. Fill greased or paper-lined muffins cups tw o-thirds full. Combine topping ingredients; sprinkle over muffins. Bake at 2070ºC for 20-25 minutes or until muffins test done. Cool 10 minutes before removin g to a wire rack. Yields 10 muffins.