Preheat the oven to 350°F. Line an 8-by-8-inch metal baking pan with parchment or foil and spray with cooking spray.

In a large microwave-safe bowl, melt the butter for about 90 seconds on high power. Stir in the granulated sugar, brown sugar, and cocoa powder. Let cool slightly before adding the vanilla, eggs, egg yolk, and corn syrup. Stir until very well combined.

Add the flour, cornstarch, and salt and stir until just combined. The batter will be thick. Pour into the prepared pan and bake for 25 minutes. Let cool completely in the pan.

For the ganache, place the cream in a small microwave-safe bowl and microwave on high for 1 minute. Add in the chocolate chips. Let stand for 3 minutes before stirring until smooth. Pour over the cooled brownies and smooth evenly.

Sprinkle with the Rainbow Candy-Coated Chocolate Chips. Refrigerate for 1 to 2 hours, or until the frosting is set, before cutting.