Preheat your oven to 350°F (175°C). Grease and lightly flour or line a 9x13 inch baking pan with parchment paper.

In a large bowl, combine the chopped rhubarb and about 1/2 cup of the dry cake mix. Toss gently to coat the rhubarb pieces evenly. This helps manage moisture and sweetness.

In a separate medium bowl, combine the remaining dry cake mix (about 1 1/2 cups) with the melted butter. Mix until crumbly, resembling coarse sand.

Press about two-thirds of the butter-cake mix mixture firmly into the bottom of the prepared baking pan to form the base crust.

Spread the coated rhubarb mixture evenly over the top of the crust layer.

Sprinkle the remaining one-third of the butter-cake mixture evenly over the rhubarb layer. Do not press down.

Bake for 30 to 35 minutes, or until the topping is golden brown and the filling is bubbly. A toothpick inserted into the center should come out mostly clean.

Let the squares cool completely in the pan on a wire rack (at least 2 hours) before slicing and serving. This allows the filling to set properly.