Mix the 1/2 cup sugar with the cornstarch and water until it is smooth. Add to cooked rhubarb, and heat just to boiling. Pour into a baking dish and dot with butter. Combine topping ingredients to make a biscuit dough. Drop by large spoonfuls over the top of the rhubarb. Sprinkle with additional sugar, if desired, and bake for 30 minutes at 400 degrees F.