Preheat of to 350 degrees. Line a 9x13 baking pan with parchment paper. Spray with non-stick cooking spray.

In a large, microwave-safe bowl, melt the butter. Stir in both sugars and the cocoa powder until well-combined. Stir in the vanilla.

Quickly whisk in the eggs, one at a time. Stir in the corn syrup, corn starch and salt and mix well. Gradually stir in the flour until combined. The batter will be thick.

Spread batter evenly in the prepared pan. Bake for 25-30 minutes until the center is set. Cool completely in the pan.

Meanwhile, make the ganache topping. Heat the cream in a large saucepan over medium-low heat until bubbles start to form around the edges. Do not let cream boil.

Remove from heat and add chocolate chips. Let stand for 1-2 minutes, then whisk until the mixture is shiny and smooth. Pour over the cooled brownies and smooth evenly. Sprinkle with rainbow chips.

Refrigerate brownies for 1-2 hours, or until frosting is set, before cutting into bars. Store in an airtight container for up to 1 week in the refrigerator.