Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving some overhang for easy removal later.

In a large mixing bowl, melt the unsalted butter in the microwave or over a double boiler. Whisk in the granulated sugar and brown sugar until well combined. Stir in the cocoa powder, salt, and vanilla extract. Allow the mixture to cool for a few minutes.

Once the mixture has cooled slightly, add the room temperature eggs one at a time, mixing until smooth and glossy after each addition.

Gently fold in the all-purpose flour and cornstarch, mixing just until no dry bits remain. Be careful not to overmix; the batter should look thick and luscious.

Pour the brownie batter into the prepared pan, smoothing the top with a spatula. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

While the brownies are baking, heat the heavy cream in a small saucepan over medium heat until simmering. Remove from heat and immediately pour over the chopped semisweet chocolate in a separate bowl. Let it sit for a minute, then whisk until smooth and glossy.

Once the brownies have cooled for about 10 minutes, pour the ganache over them, spreading it evenly. Sprinkle the rainbow candy-coated chocolate chips on top. Refrigerate for at least 1 hour to allow the ganache to set.