Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking dish with parchment paper.

Mix the brownie mix with eggs, oil, and water as per package instructions until smooth. Pour into the baking dish. Bake for 25-30 minutes until a toothpick comes out clean.

Allow the brownies to cool completely in the pan on a wire rack.

Beat softened cream cheese in a medium bowl until smooth. In another bowl, whisk chocolate pudding mix and milk until thick. Fold cream cheese into pudding and spread over cooled brownies.

Melt chocolate chips and heavy cream in a microwave-safe bowl. Pour ganache over pudding layer and spread evenly.

Sprinkle rainbow chocolate chips on top of the ganache. Cover and refrigerate for a minimum of 4 hours, preferably overnight.

Lift the dessert out of the pan using parchment overhang, slice, and serve.