Combine brown sugar and vegetable oil. Stir in egg, buttermilk, and vanilla. Add sifted dry ingredients. Stir in rhubarb and nuts. Pour into two well-greased loaf pans. Combine topping ingredients and sprinkle on top. Bake at 325 degrees about 40 minutes. Do not over bake. Remove from pans after two or three minutes and cool on a rack.