Coat the inside of a 9x13 baking dish with nonstick cooking spray.
Pour both cans of blueberry pie filling into the prepared baking dish, spreading it out evenly. Top with fresh blueberries and lemon zest.
Sprinkle the dry cake mix evenly over the blueberries. Gently smooth the surface with clean, dry hands or the back of a spatula without pressing down too hard.
Evenly distribute the thin butter slices over the top of the cake mix.
Bake for 45–55 minutes, or until the fruit is bubbling actively around the edges and the top is deep golden-brown and crisp. Cool for at least 10 minutes, then serve warm or at room temperature with vanilla ice cream.
Notes
Storage: To store leftover blueberry dump cake, allow it to cool to room temperature, then cover the baking dish. Store it in the refrigerator for up to three days. The cake crust may become soggy over time.Freezer: If you prefer to store the cake for a longer period, you can freeze it for up to two months. To freeze, allow the cake to cool completely, then wrap it tightly with plastic wrap and aluminum foil or place it in an airtight container. When you're ready to serve, thaw the cake in the refrigerator overnight before reheating it in the oven or microwave.Butter: I cut the stick of butter into 40 thin slices for even coverage. If you prefer, you can use melted butter instead and evenly pour it over the top of the cake mix.