In a large bowl;whisk together the flour, brown sugar, baking soda and the salt.In another, smaller bowl, beat together the eggs, buttermilk (or the substitute), oil and the vanilla extract. Gently stir the egg mixture into the flour mixture, just until combined. Do not over-mix.Gently fold in the chopped rhubarb and the chopped nuts.Fill greased or paper-lined muffin cups about three quarters full. Sprinkle the tops with the topping mixture.Bake at 375 ° for about 18 - 20 minutes, or until a toothpick inserted in the center comes out clean. Cool for about 10 minutes before removing the muffins from the pans.Allow to continue to cool on a wire rack.Makes 18 (large size) of the best of the best rhubarb muffins!