Preheat your oven to 350°F (175°C). Line an 8×8 inch square baking pan with parchment paper, leaving an overhang on the sides. This “sling” makes it much easier to lift the brownies out later for clean slicing. Lightly grease the parchment with non-stick spray.

In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until well combined. Add the eggs and vanilla extract. Whisk vigorously for about 1-2 minutes; this helps create that slightly shiny top and ensures the sugars are beginning to dissolve.

Sift the cocoa powder, flour, cornstarch, and salt into the wet ingredients. Sifting is vital here to prevent cocoa clumps. Use a rubber spatula to fold the mixture together gently. Stop as soon as you see no more streaks of white flour. Overmixing will lead to a cakey texture rather than the dense fudge we want.

Spread the thick batter evenly into your prepared pan. Smooth the top with a spatula. Bake for 22–25 minutes. Do not overbake! A toothpick inserted into the center should come out with a few moist crumbs attached, not clean. Let the brownies cool completely in the pan.

Place the semi-sweet chocolate chips in a medium heat-proof bowl. In a small saucepan or microwave-safe container, heat the heavy cream until it just begins to simmer. Pour the hot cream over the chocolate chips and let it sit undisturbed for 5 minutes. Gently whisk until a smooth, glossy chocolate ganache forms.

Pour the warm ganache over the cooled brownies and spread it to the edges. Immediately sprinkle the rainbow candy-coated chips over the top. Place the pan in the refrigerator for at least 1-2 hours to allow the topping to set completely. Once firm, lift the brownies out using the parchment paper and slice into squares.