Preheat & Prep: Preheat your oven to 350°F (175°C). Lightly butter or spray a 2-quart oven-safe white ceramic casserole dish; an 8x8-inch or similar size works well.

Add Blueberries: Dump the frozen raw whole blueberries straight into the prepared casserole dish. They should be evenly spread in a single, fairly even layer. It’s fine if you can see ice crystals clinging to the berries—that’s exactly how you want them.

Add Sugar: Sprinkle the granulated sugar evenly over the frozen blueberries. Don’t stir; just let the sugar sit on top so it can melt down into the berries as they bake.

Add Cake Mix: Evenly sprinkle the dry yellow cake mix over the sugared blueberries, covering the surface from edge to edge. Again, do not stir. You want a loose, even blanket of dry mix on top.

Add Butter: Slowly drizzle the melted butter over the entire surface of the dry cake mix, trying to moisten as much of the top as you can. It’s fine if a few dry spots remain; they’ll still bake up into a nice, slightly crisp topping.

Bake: Place the casserole dish on the middle rack of the preheated oven. Bake for 40–50 minutes, or until the top is golden brown and the blueberry juices are bubbling up around the edges and through a few spots in the topping.

Rest: Carefully remove the dish from the oven and let the dessert rest for at least 10–15 minutes. This cooling time helps the hot blueberry juices thicken a bit so it’s easier to spoon out.

Serve: Serve warm, scooped straight from the casserole dish into bowls. Add ice cream or whipped cream if you like, and don’t be surprised when folks go back for seconds and thirds.