Preheat the oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper, leaving some overhang for easy removal of the brownies.

In a large bowl, whisk together the flour, cornstarch, cocoa powder, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, add the eggs, sugar, butter, and vanilla. Beat on medium speed until light and fluffy.

With the mixer off, add the flour mixture. Turn the mixer on low and mix until the ingredients are well combined.

Pour the brownie batter into the prepared baking dish and bake for 28-32 minutes, or until done. Fudgy brownies are done baking when the edges appear set, but the center is still shiny or moist.

Let the brownies cool at room temperature before chilling in the refrigerator for 2 hours.

To a microwave-safe bowl, add the semisweet chocolate, milk chocolate, heavy whipping cream, and butter. Heat in 30-second intervals, stirring between each, until melted and smooth.

After the brownies have cooled, pour the chocolate ganache over the brownies, spreading it evenly.

Sprinkle the rainbow chips evenly over the chocolate ganache.

Refrigerate for about an hour, or until the chocolate is set before cutting and serving.