Preheat oven to 350°F (175°C).

Grease a 9x13 inch baking pan.

In a large bowl, mix together brown sugar and oleo until well combined.

Add the egg and buttermilk, mixing until smooth.

Gradually add the sifted flour, baking soda, vanilla, and salt, mixing until just combined.

Gently fold in the chopped rhubarb.

Pour the batter into the prepared pan.

In a small bowl, combine sugar and cinnamon.

Sprinkle the sugar-cinnamon mixture evenly over the batter.

Bake for 45 minutes, or until a toothpick inserted into the center comes out clean.

Let cool slightly before slicing and serving.