Set your oven to 350°F (175°C) so it can heat up while you prepare the batter. Lightly grease a 9×13-inch baking dish, or line it with parchment paper for easy removal later.

In a large mixing bowl, add the cake mix, eggs, vegetable oil, and milk. Beat everything together for about 2 minutes, until the batter looks smooth and well combined. Gently stir in the blueberries and chocolate chips. Pour the batter into the prepared baking dish and spread it out evenly.

In another bowl, beat the softened cream cheese until smooth. Add the melted butter, powdered sugar, and vanilla extract. Continue mixing until the texture is creamy with no lumps.

Drop spoonfuls of the cream cheese mixture over the cake batter. Using a knife, lightly swirl it through the batter to create a marbled effect. Don’t mix too much the swirls should still be visible so the cake forms its signature cracked look as it bakes.

Place the dish in the oven and bake for 45 to 50 minutes. The center should be mostly set but still a little soft. When you insert a toothpick, it should come out with moist crumbs, not wet batter. Let the cake cool before cutting and serving.