Preheat oven to 350°F. Spray a 9×13-inch baking dish with nonstick cooking spray.
To a large mixing bowl, add brown sugar, vegetable oil, eggs, buttermilk, salt, baking soda, and vanilla. Whisk to combine.
Stir in the flour and mix until just combined.
Fold in diced rhubarb. Pour the batter into the prepared baking dish.
Bake for 45-50 minutes, or until an inserted toothpick comes out with a few crumbs, but no wet batter.
Remove the cake from the oven and let it cool while you prepare the rhubarb sauce.