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.