Preheat your oven to 350°F (175°C). Line an 8x8-inch metal baking pan with parchment paper or foil, and lightly spray with cooking spray.

In a large microwave-safe bowl, melt the butter in the microwave for about 90 seconds on high power. Stir in the granulated sugar, brown sugar, and sifted cocoa powder. If the mixture is still very hot, allow it to cool slightly before adding the vanilla extract, eggs, egg yolk, and corn syrup. Stir until fully combined. Add the flour, cornstarch, and salt, and stir until just incorporated—the batter will be thick. Pour the batter into the prepared pan and bake for 25 minutes. Let the brownies cool completely in the pan.

In a small microwave-safe bowl, heat the heavy cream on high for about 1 minute. Add the chocolate chips and let the mixture sit for 3 minutes. Stir until smooth and fully combined. Pour the frosting over the cooled brownies and spread evenly. Sprinkle with rainbow candy-coated chocolate chips. Refrigerate the brownies for 1 to 2 hours, or until the frosting has set before slicing.

Brownies can be stored in an airtight container for up to 3 days at room temperature and for up to 1 week in the refrigerator. They are best served chilled.