Preheat your oven to 350°F. Line an 8-by-8-inch metal baking pan with either parchment paper or aluminum foil, then lightly coat with non-stick spray.

In a large microwave-safe bowl, melt the butter on high for around 90 seconds. Once fully melted, stir in both the granulated and brown sugars, followed by the cocoa powder. If the mixture is still too hot, let it cool down slightly before incorporating the vanilla, eggs, egg yolk, and corn syrup. Stir thoroughly until well blended. Gently fold in the flour, cornstarch, and salt until just combined, being careful not to overmix as the batter will be thick.Transfer the mixture into the prepared baking pan, spreading it evenly, and bake for about 25 minutes. Allow the brownies to cool completely in the pan before proceeding.

In a small microwave-safe bowl, heat the heavy cream on high for one minute. Add the chocolate chips to the cream and let them sit undisturbed for about 3 minutes. Afterward, stir until the mixture becomes silky smooth. Pour the frosting over the cooled brownies, smoothing the surface evenly. Decorate the top with Rainbow Candy-Coated Chocolate Chips or any toppings of your choice. Chill the brownies in the refrigerator for 1 to 2 hours, or until the frosting has fully set, before slicing.

For the best taste, store brownies in an airtight container. They can last up to 3 days at room temperature or a week in the refrigerator. These brownies are especially enjoyable when served cold.