1. Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan or springform pan and, if desired, line the bottom with parchment paper for easier removal.

2. In a large mixing bowl, combine the dry cake mix, melted and slightly cooled butter, eggs, brown sugar, and vanilla extract. Stir until a thick, soft dough forms and no streaks of dry mix remain.

3. Fold in the mini chocolate chips, most of the milk chocolate chips, and the rainbow sprinkles until they are evenly distributed throughout the dough.

4. Press the dough evenly into the prepared pan, smoothing the top with a spatula or your fingertips. Sprinkle the remaining milk chocolate chips and a few extra sprinkles over the surface.

5. Bake for 22–28 minutes, or until the edges are lightly golden and a toothpick inserted near the center comes out mostly clean with just a few moist crumbs. Do not overbake to keep the cookie cake soft and chewy.

6. Allow the cookie cake to cool completely in the pan on a wire rack. Once cool, carefully remove it from the pan and transfer to a serving plate.

7. Pipe or spoon whipped cream or whipped topping in swirls around the edge of the cookie cake, then decorate with additional rainbow sprinkles and chocolate chips.

8. Slice into 10–12 wedges and serve at room temperature.