PREHEAT THE OVEN: Set the oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper for easier cleanup.
PREP THE BERRIES: If using fresh berries, toss them with the optional sugar and let them sit for 10–15 minutes to help release juices. If using frozen berries, you can skip this step—no need to thaw.
ADD THE BERRIES TO THE PAN: Spread the mixed berries evenly in the bottom of the prepared dish. Do not add cake mix underneath the berries.
SPRINKLE THE CAKE MIX: Pour the dry cake mix evenly over the berries. Make sure it forms a loose, even layer across the top—do not stir or press down.
ADD THE BUTTER: Drizzle the melted butter slowly and evenly over the entire surface of the cake mix. Try to cover as much of the dry mix as possible. If needed, use the back of a spoon to gently press down any dry patches.
BAKE THE CAKE: Bake uncovered for 45–55 minutes, or until the top is golden brown and the berry filling is bubbling at the edges. If the top still has dry cake mix after 50 minutes, you can lightly spray those areas with a little melted butter or cover loosely with foil and bake another 5–10 minutes.
COOL AND GARNISH: Let the cake cool for 15–20 minutes. This helps it firm up slightly, especially if you want clean slices. Top with fresh berries before serving.
