Preheat the oven to 350 degrees. Lightly grease a 7-1/2 x 11 inch baking dish.
Mix together a cup of sugar and flour into a mixing bowl.
Whisk in the buttermilk, mixing well together. Pour in the melted butter and whisk well together.
Pour into the prepared baking dish.
Sprinkle the rhubarb evenly across the batter in the baking dish.
Sprinkle the remaining ΒΌ cup of sugar evenly across the batter.
Bake for 1 hour until golden. If desired, during the last 10 minutes you can sprinkle on an additional 1 teaspoon of sugar to add a nice crunch to the top.
Serve warm.