In a large bowl, combine flour, brown sugar, baking soda and salt. In a small bowl, whisk the egg, oil, buttermilk and vanilla. Stir into dry ingredients just until moistened. Fold in rhubarb and walnuts.
Fill greased or paper-lined muffin cups two-thirds full. Combine topping ingredients; sprinkle over muffins. Bake at 375° for 20-25 minutes or until a toothpick comes out clean. Cool for 5 minutes before removing from pan to a wire rack.
If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.