Preheat oven to 350°F. Line a 9x13-inch pan with parchment paper.

Whisk together flour, cocoa powder, salt, baking powder and cornstarch.

In a mixer, beat melted butter, granulated sugar and brown sugar until smooth.

Add eggs and vanilla and mix well.

Add half of the dry ingredients and mix on low. Scrape down the sides, then add the remaining dry ingredients. Mix just until combined.

Spread the thick batter evenly into the prepared pan.

Bake for 18–20 minutes. Do not overbake.

Cool completely.

For the ganache, heat both chocolate chips and heavy cream over low heat, stirring until melted and glossy.

Pour ganache over the cooled brownies and spread evenly.

Top with mini M&Ms or rainbow chocolate candies.

Refrigerate until the ganache is completely set, then cut into bars.