Preheat oven to 350°F. Spray and line a standard 9x5 in. loaf pan with a sling of parchment paper. Dice your rhubarb and set aside.
In a large bowl, whisk together the egg, buttermilk, oil, sugar, and vanilla until smooth.
Whisk in the baking powder and salt, then fold in the flour. Combine well but don’t overmix.
Fold in the rhubarb.
Turn the batter into your prepared pan. Smooth out evenly if necessary. Sprinkle a tablespoon of sugar evenly over the surface of the batter.
Bake for about 60-65 mins, or until firm when lightly touched on top and a toothpick comes out free from wet batter (wet crumbs are fine). If the bread seems to be browning too quickly towards the end of baking, lay a loose piece of foil over the top.
Let cool for 15 minutes in the pan, then use the parchment paper to remove the cake to a cooling rack to cool completely. If you aren’t using parchment paper, let the cake cool in the pan.
Notes
*If using frozen rhubarb, use it straight from the freezer and do not let it thaw.