Mix buttermilk, brown sugar, egg and oil in mixing bowl. Add flour, salt, baking soda and vanilla, and blend. Stir in rhubarb and nuts. Pour into two loaf pans that have been greased and lightly floured. Combine sugar and butter, and crumble half over top of each pan. Bake at 325 degrees for 60 minutes, or until toothpick inserted in center comes out clean.
Makes 2 loaves