Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

In a large bowl, combine the chopped rhubarb, granulated sugar, flour, and vanilla extract. Toss gently until the rhubarb is evenly coated. Spread this mixture in an even layer across the bottom of the prepared baking dish.

Sprinkle the dry yellow cake mix evenly over the entire surface of the rhubarb mixture. Do not stir or mix it in.

Distribute the cold butter cubes evenly over the dry cake mix layer. Try to cover as much of the surface as possible with the butter pieces.

Bake for 45 to 55 minutes, or until the topping is golden brown and the rhubarb underneath is bubbling and tender. If the topping browns too quickly, lightly tent with foil for the last 10 minutes.

Remove from the oven and let cool slightly for at least 15 minutes before serving. This cake is excellent served warm with vanilla ice cream or whipped cream.