Preheat oven to 350°F. Spray a 9" x 9" square baking pan with non-stick cooking spray. Then line with parchment paper, letting it extend over two sides (so you can easily pull the brownies out when they were done). Set aside.
Prepare the brownie mix according to package instructions. Pour brownie batter into prepared pan. Bake as directed. Let cool.
For the ganache: Place the coconut milk (or heavy cream) in a medium, microwaveable bowl. Microwave on high until it just begins to bubble, about 30-40 seconds. Stir in chocolate chips until melted and smooth. Let the ganache stand until cooled, about 30 minutes. Pour the frosting over the cooled brownies.
Sprinkle with rainbow mini chips. Refrigerate frosted brownies until set, about an hour. Make sure you chill the brownies before cutting into squares to get a clean cut. If you can't wait, you can cut into them before chilling but just know that the ganache frosting won't be set and it might be a little messy. Remove chilled brownies from pan by lifting up and out with parchment paper. Cut into 16 squares using a sharp knife. Serve and enjoy!