Combine brown sugar, oil and egg in a large bow. In another bowl, combine buttermilk, baking soda, salt and vanilla and add to large bowl with flour. Fold in rhubarb and pecans. Pour into two 8x4x3-inch bread pans which have been sprayed with Pam. Sprinkle loaves with sugar combined with butter.
Bake at 325 degrees for about 1 hour or until tester comes out clean. Run a butter knive around loaves and turn out when cool. Makes 2 loaves. (Judy Pike)