Directions
Preheat your oven to 350°F (175°C). Set out a 9×13-inch glass baking dish; there’s no need to grease it, as the filling and butter will keep things from sticking.
Spoon the rhubarb pie filling into the baking dish and spread it into an even layer, making sure it reaches all the corners. This will give you that pretty pink bubbling edge when it bakes.
Sprinkle the dry yellow cake mix evenly over the rhubarb filling. Use your hands or the back of a spoon to gently spread it so there are no big bare spots, but don’t stir it in—you want distinct layers.
If using, sprinkle the granulated sugar and cinnamon lightly over the dry cake mix for a touch of extra sweetness and warmth. This step is optional but adds a cozy, Nana-style touch.
Slowly drizzle the melted butter evenly over the entire surface of the cake mix. Try to cover as much of the dry mix as you can so the topping bakes up crisp and golden. A few dry patches are fine; they’ll turn into crunchy bits.
Place the baking dish on the middle rack of the preheated oven. Bake for 35–45 minutes, or until the top is golden brown, the edges are bubbling with pink rhubarb, and the center looks set with no obviously wet cake mix.
Remove the dish from the oven and let the dump cake cool for at least 10–15 minutes. This resting time helps the filling thicken up a bit while still staying warm and cozy.
Serve warm, scooping down through the crisp buttery topping to get plenty of tart rhubarb in each portion. Add ice cream or whipped cream if you like, and enjoy straight from the glass dish on the counter, just like Nana Pat would.
