Stir together in medium bowl the buttermilk brown sugar egg oil and vanilla and then add to dry ingredients stirring until just combined Fold in rhubarb and nuts
Pour batter into 13 x 9 x 2-inch baking pan
Sprinkle with mixture of 12 cup granulated sugar cinnamon and butter which you combined by stirring with a fork
Bake at 350 for 40 minutes or until toothpick inserted in center comes out clean