Spray a 9x5x3-inchglass baking pan with Baker's Release spray or liberally grease the pan.
Chop rhubarb and set aside. Do not use any of the leaves.
Stir together the butter with sugar then add the eggs, buttermilk and vanilla.
Mix the flour, baking powder, baking soda, salt and cinnamon until smooth. Toss in the rhubarb to coat, then gradually stir the flour/rhubarb into the wet mix. Only stir enough to combine- some specks of flour are ok in the batter.
Pour batter into the prepared pan and baking for 65 minutes or until a toothpick tests clean when inserted into the center.
Cool in the pan for ten minutes before turning out the loaf and cooling completely on a wire rack.