1Preheat oven to 350 degrees F (175 degrees C). Prepare a 11x7x2-inch baking dish with non-stick cooking spray.
2Spread rhubarb into the baking dish. Sprinkle white sugar, gelatin mix, and yellow cake mix over the rhubarb. Pour water over the rhubarb mixture. Drizzle butter over the rhubarb mixture.
3Bake in preheated oven until rhubarb is tender, about 50 minutes. Cool on a wire rack for 15 minutes. Dust with confectioners' sugar and top with whipped topping.