Preheat oven to 350° (325° if you're using a glass pan). Place rhubarb in a bowl and sprinkle with sugar. Let stand while preparing the batter.
Spray an 8-inch baking pan with nonstick spray.
Cream together the butter and sugar in an electric mixer, until fluffy. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla.
In another bowl, mix together the dry ingredients until uniformly blended. Add the dry ingredients to the butter mixture, alternating with the buttermilk. Stir thoroughly, but minimally, until combined, don't overmix.
Fold in the rhubarb and transfer the batter to the prepared pan. The batter will be stiff and you'll have to work with it a little, spreading it in the pan. Don't worry if it doesn't look completely uniform in the pan.
Mix together the brown sugar and cinnamon and sprinkle on top of the batter.
Bake 25 minutes, or until a toothpick inserted all the way into the center comes out clean.