Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving a little overhang on the sides to make removing the brownies easier later.

In a large mixing bowl, whisk together the melted butter and sugar until smooth and glossy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until everything is fully combined.

Sift in the cocoa powder, flour, and salt. Gently fold the ingredients together until a thick brownie batter forms. Avoid overmixing, as this can make the brownies less fudgy.

Pour the batter into the prepared pan and spread it evenly. Bake for 20–25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let the brownies cool completely in the pan.

Place the chocolate chips in a heatproof bowl. Heat the heavy cream in a small saucepan until it begins to steam (but not boil). Pour the hot cream over the chocolate chips and let it sit for about one minute, then stir until smooth and glossy.

Pour the ganache over the cooled brownies and spread it evenly with a spatula. Immediately sprinkle the rainbow candy-coated chocolate chips over the top.

Place the brownies in the refrigerator for 30–45 minutes to allow the ganache to set. Once firm, lift the brownies out using the parchment paper and cut them into squares.