Ingredients:

  • 4 cups fresh or frozen blueberries (about 560 g)
  • 1/2 cup granulated sugar (100 g) — reduce to 1/3 cup if berries are very sweet
  • 2 tablespoons cornstarch (16 g) — or 2 tbsp tapioca starch
  • 1 tablespoon fresh lemon juice (15 ml)
  • 1 teaspoon lemon zest (optional, about 2–3 g)
  • 1/2 teaspoon pure vanilla extract (2.5 ml)
  • Pinch of fine salt (about 1/8 teaspoon)
  • 1 (15.25 oz / ~432 g) box yellow cake mix (use gluten-free mix if needed)
  • 1/2 cup (1 stick / 113 g) unsalted butter, melted — or 8 tablespoons (if using salted butter, omit the pinch of salt above)
  • Optional: 1/4 cup old-fashioned oats (20 g) or 2 tablespoons coarse sugar for extra crunch
  • Optional garnish: vanilla ice cream, whipped cream, or lemon zest

Instructions:

  1. Preheat oven to 350°F (175°C). Lightly grease a 9x13-inch (23x33 cm) baking dish or spray with nonstick spray.
  2. In a large bowl, combine the blueberries, granulated sugar, cornstarch, lemon juice, lemon zest (if using), vanilla extract, and a pinch of salt. Toss until berries are evenly coated and the cornstarch is dissolved. If using frozen berries, use them straight from frozen without thawing.
  3. Pour the blueberry mixture into the prepared baking dish and spread into an even layer.
  4. Evenly sprinkle the dry yellow cake mix over the blueberry layer (do not mix). Scatter the oats or coarse sugar over the cake mix if using.
  5. Drizzle the melted butter evenly over the top of the cake mix. Pour in several spots and gently spread a little with a spatula or the back of a spoon so the butter covers most of the surface; small pools of butter are fine.
  6. Bake in the preheated oven for 40–50 minutes, until the filling is bubbling around the edges and the topping is golden brown. If the top browns too quickly before the filling is bubbling, tent loosely with foil and continue baking until bubbling.
  7. Remove from the oven and let rest 10–15 minutes to allow the filling to thicken. Serve warm with vanilla ice cream, whipped cream, or a sprinkle of lemon zest. Reheat leftovers gently in the oven.