Preheat your oven to 350°F. While it heats, grease a 9x13 inch baking dish (or similar size) with butter or cooking spray, making sure to coat the bottom and sides evenly. This prevents the cake from sticking and ensures even browning on the edges.

In the prepared baking dish, combine the blueberries, sugar, lemon zest, vanilla extract, and salt. Stir gently until the berries are evenly coated with the sugar mixture—this helps the blueberries release their juices and creates a jammy filling. I like to taste a berry at this point to make sure the sweetness is balanced; if your blueberries are very tart, you can add an extra tablespoon of sugar.

Sprinkle the dry lemon cake mix evenly over the blueberry mixture, covering it completely but don't stir—you want the cake mix to stay on top. Then pour the melted and cooled butter evenly over the cake mix layer. I find it easiest to pour the butter in a thin stream back and forth across the pan to ensure it distributes evenly and reaches all the dry mix.

Place the baking dish in the preheated oven and bake for 1 hour, or until the top is golden brown and a toothpick inserted into the cake portion comes out clean or with just a few moist crumbs. The blueberries will bubble up around the edges slightly—this is exactly what you want and signals that the filling is cooked through.

Remove the cake from the oven and let it cool for 10-15 minutes on a wire rack or trivet. This allows the filling to set slightly while the cake stays warm and tender. The cake will continue to firm up as it cools, making it easier to serve. Scoop into bowls and serve warm with vanilla ice cream or whipped cream if desired.