Preheat oven to 350ºF. Lightly spray a 9×13-inch pan with cooking spray.: As the oven warms you will notice a dry, anticipatory heat that promises browning; preheating ensures the butter begins melting immediately and the top crisps rather than simply soaking in. The pan should be evenly coated so the filling does not stick when serving. A common mistake is skipping preheat which results in a bland, underbaked topping and uneven bubbling; always wait until the oven reaches temperature before sliding the pan in.
Pour blueberries into pan, sprinkle with sugar and lemon zest.: You will see deep violet berries glisten as the sugar clings to them and the lemon zest sparkles like tiny citrus confetti. As the fruit warms in the oven it will loosen and sigh into syrup, so the sugar is key to coax those juices out. Avoid overmixing, which can crush the berries and make the filling runny; gentle tossing preserves some whole berries for pleasing texture.
Sprinkle dry cake mix evenly over blueberries.: The dry cake mix should sit like a snowfall over the fruit, forming a dry barrier that will brown and crackle in the heat. Even coverage is the visual cue you are aiming for, because gaps lead to exposed bubbling fruit and clumps. If the mix is applied unevenly you may end up with raw pockets or overly dense areas; take a moment to pat it into a uniform layer without pressing down.
Cover cake mix with butter slices, trying to cover as much of the cake as possible.: Watch the transformation as the cold butter melts, creating steaming golden pools that soak the dry mix and encourage crisping. The sizzling begins almost immediately, and the aroma shifts from sugary to buttery, signaling progress. If your butter is too melted when placed on top, the topping can become greasy instead of flakey; keep slices cold and distribute them evenly for best texture.
Bake for 25 to 30 minutes, until the cake mix is golden brown.: During baking you will hear a gentle bubbling from the fruit and see steam rising, while the top transitions to a warm, golden brown. That color is your indicator of readiness, and the edges will smell nutty and caramelized. A common error is removing it too soon; an underbaked top will be pale and soft, while overbaking can dry the edges. Aim for a glossy, bubbling center and uniformly browned top.
