Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper.
In a large bowl, whisk together melted butter, granulated sugar, and brown sugar until well combined.
Add eggs and vanilla extract, mixing until fully incorporated.
Sift in cocoa powder, flour, and salt, mixing just until combined. Avoid overmixing.
Pour the batter into the prepared pan, spreading evenly.
Bake for 25-30 minutes, or until a toothpick inserted in the center comes out mostly clean.
Let brownies cool completely in the pan on a wire rack.
For the ganache: Place chocolate chips in a bowl. Heat heavy cream in a pan on the stovetop until it just begins to simmer, then pour over chocolate chips. Let sit for 3 minutes, then stir until smooth.
Spread ganache over cooled brownies. Sprinkle rainbow candy-coated chocolate chips on top.
Refrigerate for about 30 minutes to set the ganache before slicing into squares.