Preheat oven to 350°F (175°C) and grease a 9×5-inch loaf pan.
In a medium bowl, whisk together flour, baking soda, baking powder, cinnamon, and salt.
In a large bowl, whisk egg, oil, ½ cup brown sugar, sugar, sour cream, and vanilla until smooth.
Add dry ingredients to wet ingredients and stir until just combined.
Fold in chopped rhubarb gently.
Pour batter into the loaf pan and spread evenly.
Sprinkle remaining 1 tablespoon brown sugar on top.
Bake the Rhubarb Bread for 50 minutes or until a toothpick comes out clean.
Cool in pan for 10 minutes, then transfer to a wire rack to cool completely.