Preheat the oven to 350°F (180°C). Line a 9x13-inch baking pan with foil or parchment paper, making sure to leave some overhang for easy removal. If using foil, spray it well with nonstick cooking spray, and set aside.

In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until well combined. Then whisk in the eggs, egg yolk, and vanilla extract until fully combined.

Sift in the cocoa powder, flour, and salt and continue whisking until just combined, stopping to scrape down the sides of the bowl as needed.

Scoop the batter into the prepared baking pan and spread it around into one even layer. Bake for 27 to 33 minutes or until a toothpick inserted into the center comes out clean with just a few moist crumbs.

Remove from the oven and allow to cool completely.

To make the ganache: Place the chopped chocolate and corn syrup into a large heatproof bowl. Set aside.

Heat the heavy cream in a medium pot and bring to a simmer on the stove (or heat in the microwave for 45 seconds to 1 minute).

Pour the simmering cream over the chocolate and let it sit for 3 minutes.

Mix with a rubber spatula or spoon until the mixture is smooth and well combined. If any chocolate clumps remain, microwave in 20 to 30-second increments until melted and smooth.

Use a spatula or spoon to spread the chocolate ganache over the cooled brownies.

Sprinkle with rainbow candy coated chocolate chips, mini M&Ms, or sprinkles. Refrigerate for 1 to 2 hours to allow the ganache to set before serving. Cut into squares when ready to serve!