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Preheat the oven to 325°F (163°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang for easy removal.
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In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth.
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Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
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In a separate bowl, whisk together the cocoa powder, flour, cornstarch, and salt.
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Fold the dry ingredients into the wet ingredients until no dry streaks remain. Do not overmix.
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Spread the batter evenly into the prepared baking pan.
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Bake for 28–34 minutes, or until the center is set and a toothpick inserted in the middle comes out with a few moist crumbs.
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Cool the brownies completely in the pan.
Make the Ganache
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Place the chocolate chips into a heatproof bowl.
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Heat the heavy cream until it just begins to simmer.
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Pour the hot cream over the chocolate chips and let stand for 2 minutes.
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Stir until smooth, then mix in the butter until glossy.
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Spread the ganache evenly over the cooled brownies.
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Immediately sprinkle the rainbow candy-coated chocolate chips over the ganache.
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Refrigerate for about 1 hour before slicing into squares.
